Stratigraphic and lithofacies analysis of the Gohatsion Formation in the Blue Nile basin, central Ethiopia: Implications for depositional setting

Journal Pre-proof Stratigraphic and lithofacies analysis of the Gohatsion Formation in the Blue Nile basin, central Ethiopia: Implications for depositional setting Samuel G. Chernet, Balemwal Atnafu, Asfawossen Asrat PII:

S1464-343X(19)30348-6

DOI:

https://doi.org/10.1016/j.jafrearsci.2019.103693

Reference:

AES 103693

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Journal of African Earth Sciences

Received Date: 9 April 2019 Revised Date:

30 October 2019

Accepted Date: 30 October 2019

Please cite this article as: Chernet, S.G., Atnafu, B., Asrat, A., Stratigraphic and lithofacies analysis of the Gohatsion Formation in the Blue Nile basin, central Ethiopia: Implications for depositional setting, Journal of African Earth Sciences (2019), doi: https://doi.org/10.1016/j.jafrearsci.2019.103693. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Stratigraphic and lithofacies analysis of the Gohatsion Formation in the Blue Nile

2

Basin, central Ethiopia: Implications for depositional setting

3 4

Samuel G. Chernet*, Balemwal Atnafu, Asfawossen Asrat

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School of Earth Sciences, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia

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*Corresponding author: Tel.: +251 913831118; Fax: +251 11 1239462. E-mail:

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[email protected] (S.G. Chernet)

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Abstract

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The Gohatsion Formation consists of a cyclic intercalation of fine siliciclastic and

14

evaporite/carbonate beds and has been subdivided into three informal members, namely the

15

Lower Mudrock Member, the Gypsum Member and the Upper Mudrock Member. Four

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lithostratigraphic sections were constructed at the Gohatsion, Dejen, Mugher and Jemma

17

localities. Lithofacies analysis of the succession at outcrop and petrographic scales indicate a

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broad tidal belt influenced by seasonal and regional climate and sea level changes. Integrated

19

lithofacies depositional models for the corresponding informal members have been proposed

20

in order to demonstrate the lithological response to local and global climate change and sea

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level fluctuations that occurred during the initial phase of the Tethyan Sea transgression

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during the early to middle Jurassic. Lithofacies of the Lower Mudrock Member show

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deposition within a siliciclast-dominated back barrier tidal flat system. Deposition of the

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Gypsum Member occurred in a marine coastal sabkha belt, whereas the Upper Mudrock

25

Member was deposited in a broad coastal shelf prone to seasonal marine flooding. The

1

26

models show that all the members of the Gohatsion Formation were formed within a broad

27

coastal tidal belt with continental influence and sediment delivery from the northwest, and a

28

slowly encroaching transgressive marine influence from the southeast.

29 30

Keywords

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Gohatsion Formation, Blue Nile Basin, Tidal flats, Sea margin sabkha belt, Lithofacies

33

analysis, Evaporite, Depositional setting

34 35

1. Introduction

36 37

Sedimentary basins are results of large-scale amalgamation of depositional systems. The

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distribution and stacking pattern of depositional systems in a sedimentary basin is controlled

39

by global, regional and local factors such as tectonics, climate, eustasy, as well as type and

40

supply of sediments (Beerbower, 1964). The sedimentary history of the Blue Nile Basin

41

(BNB) in Ethiopia has been of great interest in the past few decades (e.g., Kazmin, 1975;

42

Assefa, 1981; Russo et al., 1994; Gani et al., 2008; Wolela, 2008, 2014; Dawit and Bussert,

43

2009; Dawit, 2010, 2016).

44 45

The Gohatsion Formation in the Blue Nile Basin represents a complex transitional

46

depositional system in a semi-arid peritidal depositional setting (Assefa, 1981; Russo et al.,

47

1994; Dawit, 2010). It is overlain by a Callovian-Lower Kimmeridgian calcareous unit

48

(Antalo Limestone) and underlain by the Permo-Triassic Adigrat Sandstone (Assefa, 1981;

49

Russo et al., 1994; Dawit and Bussert, 2009; Dawit, 2014). Assefa (1981) assigned Liassic to

2

50

late Bathonian age to this formation on the basis of diagnostic foraminifera (e.g.,

51

Nauliloculina circularis), and stromatoporodia (e.g., Cladocoropsis mirabilis Felix).

52

Stratigraphic position and description of the Gohatsion Formation, in the context of the

53

Paleozoic-Mesozoic succession of the basin, has been revised several times, prior to the work

54

of Assefa (1981). Krenkel (1926) named this unit the ‘strata of Abbay’ while Jespen and

55

Athearn (1961) used the name ‘Shale and gypsum unit’. Kazmin (1975) categorized the

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Gohatsion Formation as part of the “Antalo Group” which comprises Abbay Beds (Gohatsion

57

Formation), Antalo Limestone and Agula Shale. Some stratigraphic modifications including

58

major ones have also been suggested after the work of Assefa (1981). Wolela (2008) added a

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50 meter ‘transitional unit’ between the contacts of the Adigrat Sandstone and Gohatsion

60

Formation. Gani et al. (2008), reclassified the Antalo Limestone and Gohatsion Formation as

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Upper Limestone, Lower Limestone and Glauconitic Sandy Mudstone units. These works,

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however, did not elaborate the reasons for revising the stratigraphy.

63 64

The first formal lithostratigraphic and biostratigraphic description was given by Assefa

65

(1981), wherein a formal type section was assigned to the Gohatsion locality. Lithofacies of

66

this unit includes cyclic inter-bedding of bioturbated mudstone, siltstone, shale, marlstone,

67

gypsum and dolostone (Assefa, 1981; Russo et al., 1994; Wolela, 2008; Dawit, 2010). Assefa

68

(1981) attributed a broad range of depositional environments within a peritidal setting for the

69

Formation but did not describe their spatial arrangement and stacking pattern in the basin.

70

Accordingly, the Gohatsion Formation, at least in its type section, was informally sub-divided

71

into four sub-members, namely the Mudstone Member, Lower Claystone Member, Gypsum

72

Member and Upper Claystone Member (Assefa, 1981). However, such subdivision was

73

strictly applied only to the type area by Assefa (1981) and few other subsequent works (e.g.,

74

Russo et al., 1994). In addition, the lithofacies description and interpretation of Assefa (1981)

3

75

lacked some important details especially concerning the evaporite successions. The current

76

work, therefore, reexamines and expands the work of Assefa (1981) on the stratigraphic and

77

lithofacies description of the Gohatsion Formation.

78 79

The main objectives of this work are to: (i) establish the regional characteristics of the

80

Formation based on several lithostratigraphic sections constructed from outside the type area

81

(i.e., outside the Gohatsion locality according to Assefa, 1981), (ii) conduct detailed

82

lithofacies analysis at outcrop and microscopic scales, and (iii) propose a depositional model

83

for the Formation.

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2. Geological Setting

85 86

The breakup of Gondwana during the Paleozoic-Mesozoic era can be summarized in two

87

stages. The first stage (rifting stage) lasted form 300-205 Ma (Schandelmeier and Rynolds,

88

1997; Scotese et al., 1999; Golonka and Ford, 2000; Wolela, 2014), when a large

89

intracontinental rift (“the Karoo rift”), formed along the borders of the present day eastern

90

African passive margin (Binks and Fairhead, 1991; Worku and Astin, 1992; Hankel, 1994;

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Schandelmeier and Rynolds, 1997; Scotese et al., 1999; Golonka and Ford, 2000; Wolela

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2014; see Fig. 1A and B). The second stage lasted form 205-157 Ma (Worku and Astin,

93

1997; Wolela, 2014). At this stage, Gondwana disintegrated into several blocks that started to

94

drift apart (Schandelmeier and Rynolds, 1997; Scotese et al. 1999; Golonka, 2007; Wolela,

95

2014). The rift to drift transition was also facilitated by transgressions caused by the Jurassic

96

sea level highstand (Haqet al., 1988; Schandelmeier and Rynolds, 1997; Russo et al., 1994).

97 98

In the current Ethiopian borders, the intracontinental rift stage resulted in the formation of the

99

Ogaden, Blue Nile and Mekele basins (Beyth, 1972; Worku and Astin, 1992; Russo et al.,

4

100

1994; Schandelmeier and Rynolds, 1997; Hunegnaw et al., 1998;Wolela, 2014). Some arms

101

of the Karoo rift such as the Mekele and Blue Nile basins did not fully develop beyond the

102

rifting stage and remained northwest trending ‘failed arms’ (Bosellini, 1992; Russo et al.,

103

1994; Tadesse et al., 2003; Gani et al., 2008). Over the period of 400 My, these rift basins

104

served as depo-centers for thick sedimentary sequences representing both continental and

105

marine environments (Beyth, 1972; Assefa, 1991; Worku and Astin, 1992; Russo et al., 1994;

106

Dawit and Bussert, 2009; Wolela 2014).

107 108

According to Bosellini (1992) and Geleta (1997), three major transgression and regression cycles

109

occurred in eastern Africa during the Mesozoic era. The first major transgression-regression

110

event was in response to the initial stages of the breakup of Gondwanaland during middle to

111

late Jurassic (Bosellini, 1992).The majority of Mesozoic sediments in the Blue Nile basin are

112

results of the first transgression and regression cycle which includes the Gohatsion Formation

113

(Russo et al., 1994). The second major transgression event took place during the Aptian, but

114

was mostly limited to the Ogaden Basin, where it formed the Mustahil and Ferfer Formations

115

(Bosellini, 1992; Mateer et al., 1992). The third minor transgression cycle also occurred in

116

the Ogaden Basin and parts of the Southeastern Ethiopian Plateau during late Cretaceous to

117

middle Eocene. It was responsible for the formation of the Taleh and Kerker Formations

118

(Geleta, 1997; Hunegnaw et al., 1998; Atnafu and Kidane, 2012).

119 120

[Insert Fig. 1 here]

121

[Insert Fig. 2 here]

122 123

Paleozoic-Mesozoic sediments of the Blue Nile Basin have been informally categorized into

124

8 units (Assefa, 1981; Russo et al., 1994; Dawit and Bussert, 2009; Dawit, 2010, 2016;

5

125

Wolela, 2014; see Fig. 1C and Fig. 2). The Pre-Adigrat I, II, and III Formations represent

126

sedimentation at the initial stages of intracratonic basin formation during late Permian

127

(Schandelmeier et al., 1997; Dawit and Bussert, 2009). These three units represent

128

sedimentation under various depositional environments including floodplain, crevasse-splay,

129

aeolian and alluvial flood plain systems (Dawit and Bussert, 2009).

130 131

The Triassic to Middle Jurassic Adigrat Sandstone Formation forms an unconformable

132

contact with either the Pre-Adigrat units or the Precambrian basement rocks (Dawit and

133

Bussert, 2009; Wolela, 2009, 2014). Earlier works such as Russo et al. (1994) and Wolela

134

(2008) interpreted this unit to be of purely fluviatile origin. In contrast, recent works of Dawit

135

and Bussert (2009) and Dawit (2010, 2016) proposed a storm dominated shore face to a

136

barrier/inlet spit as depositional setting for this unit. Furthermore, Dawit (2016) outlined three

137

unconformity bounded stratigraphic units within this succession justified by the presence of

138

paleosol layers.

139 140

The Gohatsion Formation and the overlying Antalo Limestone, collectively referred to as the

141

Hammanlei Formation by Wolela (2014), represent a transgressive phase and flooding of the

142

East African craton (Russo et al., 1994; Dawit and Bussert, 2009; Wolela, 2014).

143 144

The retreat of the transgressive Neo-Tethyan Sea shifted the sedimentation style from marine

145

to continental as documented in the lower Cretaceous Mugher Mudstone and Barremian-

146

Cenomanian Debre Libanos Sandstone Formations (Assefa, 1991; Wolela, 2009, 2014).

147 148

The 2000 m thick sedimentary succession of the Blue Nile Basin is topped by the Oligocene

149

flood basalts with a maximum thickness reaching up to 1000 meters (Abbate et al., 2015).

6

150

Such volcanic activity is believed to be in response to the breakup of the Arabian plate from

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eastern Africa and the formation of the Red Sea (Hofmann et al., 1997; Pik et al., 1998;

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Kieffer et al., 2004; Abbate et al., 2015).

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3. Materials and Methods

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The study involved construction of detailed lithostratigraphic sections at four localities,

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namely at Gohatsion, Dejen, Jemma and Mugher areas accompanied by detailed geological

158

mapping (Fig.3). Detailed logging and description of the lithostratigraphic and

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sedimentological features was conducted at the four localities (Fig. 4) where the rocks are

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exposed at stream, road and quarry cuts. Systematic variation in lithostratigraphy was used to

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re-categorize individual members into the Lower Mudrock, Gypsum and the Upper Mudrock

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Members. A total of 15 samples were collected from the Gohatsion and Dejen localities for

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petrographic analysis. Transmitted light microscopy was used to perform textural and

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mineralogical analysis on resin-impregnated thin sections. The detailed field logging and

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description of various sedimentary textures and structures were integrated with the

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petrographic microfacies analysis. The lithofacies description and association have been

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compared and checked against previous works including those of Assefa (1981); Dawit and

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Bussert (2009); Flemming (2010), Wolela (2014), and Warren (2016).

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[Insert Fig. 3 here]

171 172

4. Results

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4.1. Outcrop Descriptions

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175 176

4.1.1. Mugher Area

177 178

The Mugher section was logged in an active gypsum quarry along the bank of Sodoble River

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(Fig. 3A). In this section, the 184 m thick Gohatsion Formation is conformably overlain and

180

underlain by the Antalo Limestone and the Adigrat Sandstone Formations, respectively (Fig.

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4A). The Gypsum Member in this section is significantly thinner compared to the other

182

measured sections. The Upper and Lower Mudrock Members also contain abundant

183

dolomitic limestone layers (0.5-1 m thick on average).

184 185

4.1.2. Gohatsion-Dejen Area

186 187

The Gohatsion and Dejen sections are exposed by the deeply incised canyons of the Abay

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River (Fig. 3B).The area also represents the central part of the BNB where the thickest

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succession of the Gohatsion Formation is recorded. This area was also where the type section

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of the formation was measured and described (Assefa, 1981). The total measured sections at

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the Gohatsion and Dejen localities are 433 and 398 m, respectively (Fig. 4B and C). In both

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sections, the Gohatsion Formation is conformably bounded by the Antalo Limestone and the

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Adigrat Sandstone formations.

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4.1.3. Jemma Area

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This section was logged along the banks of the Jemma River (Fig. 3C). Unlike the other

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measured sections this area is affected by significant reworking due to either tectonics or land

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sliding, hindering construction of a detailed section. Nevertheless, a ~400 m thick section was

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constructed by using river cut, quarry and road outcrops. The thickness of the upper and

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Lower Mudrock Members is significantly higher compared to the other sections (Fig.4D).

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[Insert Fig. 4 here]

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4.2. Lithofacies analysis

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A total of 14 lithofacies, categorized into siliciclastic and evaporite facies types, have been

207

recognized in the studied sections and are described below.

208 209

4.2.1. Siliciclastic facies

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4.2.1.1. Facies S1 (mudstone and siltstone facies)

212 213

This facies is observed in the Lower Mudrock Member and is composed predominantly of

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mudstone and siltstone successions. This lithofacies was observed in the Dejen locality. Both

215

the siltstone (0.15-1 m thick on average) and the mudstone (few millimeters to 1.5 m on

216

average) are partly calcareous. The major sedimentary features observed are heterolithic

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bedding, channel scour accompanied by flute cast structures (Fig. 5A and B), bidirectional

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(sinuous) ripple cross-laminations and linguoid ripple marks. The massive mudstone beds in

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between and towards the bottom of the successions also show mottled colors ranging from

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brilliant red to yellowish gray and dominantly olive-green. Green mudstone units and some

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siltstone beds are intensely bioturbated. This lithofacies shows a gradational contact with

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either lithofacies S5 or E6 (described below).

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The presence of heterolithic bedding along with ripple marks and cross-lamination with

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ample bioturbation suggests deposition under tidal mudflat environment. Additionally, the

226

presence of migrating channel scours with flute cast structure suggests seasonal meandering

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channels draining into the intertidal zone (Flemming, 2010). Interplay of colors in mudstone

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and shale indicates seasonal variation in clastic input, resulting in a change in the oxidation

229

conditions that ultimately control organic matter decomposition that enables the color change

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(Schieber, 1999; Potter et al., 2005).

231 232

4.2.1.2. Facies S2 (siltstone-sandstone-mudstone facies)

233 234

This lithofacies type is similar to lithofacies type S1. However, the presence of sandstone

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interlayered with siltstone and mudstone, thin planar cross-lamination in siltstone, lenticular

236

and wavy bedding (Fig.5C) with relatively moderate bioturbation makes it different from S1.

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Large scale co-sets of planar cross-laminated siltstone have 4 m thickness with individual

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small scale sets reaching up to 0.5 m thickness, while the sandstone layers have an average

239

thickness of 0.5 m. Siltstone beds found in the Gohatsion locality, close to the contact

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between the Lower Mudrock Member and the Gypsum Member, are mica rich and show

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interference ripples along with bioturbation (Fig. 5D). This lithofacies shares gradational

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contacts with the lithofacies S5, S4, S3 and a sharp contact with E6. The siltstones are

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dominated by muscovite (~60%), along with quartz and plagioclase feldspars (~25%).

244

Detrital clay, autogenetic quartz, calcite and opaque minerals serve as matrix (~15%). Thin

245

irregular micro-laminations form patchy and scoured contact surfaces between quartz

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dominated and clay and muscovite dominated horizons.

247

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248

This lithofacies is interpreted as an offshore extension of facies S1 in a tidal mudflat system.

249

According to Flemming (2010), siliciclastic rocks deposited in tidal mudflats show

250

progressive increase in grainsize towards offshore. The increase in grainsize coupled with the

251

presence of cross-laminated siltstone and typical tidal lithofacies described in facies S1

252

indicate a mixed tidal flat setting. In siltstones, the presence of thin irregular micro-

253

laminations with scoured contact between the mica dominated and quartz dominated layers is

254

also indicative of deposition under weak tidal currents (Potter et al., 2005).

255 256

4.2.1.3.Facies S3 (planar cross-bedded sandstone facies)

257 258

This lithofacies is observed only in the Gohatsion and Jemma localities, within the Lower

259

Mudrock Member. It is dominated by multiple stacks of silty sandstone with subordinate

260

mudstone lenses forming flaser bedding (Fig.5E). The average thickness of this facies

261

succession is 4-5 m in the measured sections. The top of the succession is formed by thick

262

beds of sandstone (0.9 m) having planar cross-bedding while thin beds at the base of the

263

succession show migrating ripple marks. Co-sets of planar cross-bedded layers have

264

thicknesses ranging from 0.3-0.5 m. Towards the top of the succession, cross-laminated sand

265

beds with sub-horizontal sand and mud lenses in the form of flaser beds are present. The

266

sandstone is calcareous. The average modal proportion of the sandstones is: quartz (58-80%),

267

plagioclase and microcline feldspars (3-5%), and muscovite (0-8%) and rock fragments (1-

268

9%). The matrix is composed of calcite cement (15-30%) and minor detrital clay (>2%).

269

Monocrystalline quartz is dominant in all samples while polycrystalline quartz is rare. Rock

270

fragments are dominantly metamorphic/plutonic origin while sedimentary rock fragments

271

(chert) are rare. Traces of rutile and zircon are found included within monocrystalline quartz

272

grains. Grains are moderately rounded to well-rounded and well sorted (Fig. 5F). Calcite

11

273

cement exists in either microcrystalline or poikilotopic form. Dissolution and leaching of

274

feldspar and rare quartz grains can also be seen. All the analyzed samples are quartz arenites

275

(after Folk, 1951).

276 277

The presence of multiple stacks of planar cross-bedded sandstone with flaser beds of

278

mudstone are typical of exposed sand flats and barrier islands (Flemming, 2010). Quartz

279

arenites are products of extended periods of reworking (Tucker, 2001). Furthermore, well

280

rounded and sorted texture is indicative of wave reworking and sorting.

281 282

[Insert Fig. 5 here]

283 284

4.2.1.4.Facies S4 (gypsiferous shale and pedogenic gypsum facies)

285 286

This facies is observed in all localities commonly associated with the Gypsum Member. It is

287

composed of gypsiferous shale interlayered with pedogenic gypsum forming gypcretes and

288

gypsum crusts. Uniformly thin (0.2 m thick) beds of shale showing mottled color are often

289

found interlayered with gypsiferous shale with an average thickness of 0.4-0.8 m.

290

Intercalation of sheets of fenestric dolomite and algal laminates is also common, especially at

291

the Dejen and Gohatsion sections. This lithofacies forms a gradational contact with S2 and

292

sharp contact with E7, E6, E2 and E1.

293 294

A pedogenic gypsum soil profile commonly forms in areas surrounding saline water bodies in

295

arid areas (Aref, 2003). They could also form by capillary evaporation (Warren, 2016).

296

Interlayering of algal laminates and fenestric dolomite beds along with mottled gypsiferous

297

shale beds suggests a supratidal setting.

12

298 299

4.2.1.5. Facies S5 (shale and mudstone facies)

300 301

This is the most common lithofacies in the upper and Lower Mudrock Members of all the

302

logged sections. Though the dominant lithologies are shale and mudstone, subordinate beds

303

of siltstone and claystone are also found randomly distributed within the succession. In the

304

Mugher locality, thin dolomitic limestone beds with an average thickness of 0.5 m also occur.

305

Shale and mudstone form massive beds ranging from 1 to 6 m. The composite thickness of

306

massive shale and mudstones reaches up to 15 m in the Gohatsion and Dejen localities.

307

Localized slumps and contorted beds are very common in the Dejen and Jemma localities.

308

Thin (0.2-0.5 m) claystone beds forming wavy, parallel, and discontinuous laminations are

309

also found within mudstone beds. Apart from laminations, claystone beds also show flame

310

structures and rip-up clay clasts within siltstone and mudstone beds. Siltstone units show

311

dominantly parallel lamination with a fining-upwards bedding (Fig. 6A). The shale and

312

mudstone beds show olive green or dark red color when they are not mottled and are partly

313

calcareous.

314 315

Dominance of fine-grained sediments forming massive beds is indicative of a lesser energy

316

environment. According to Potter et al. (2005), deposition of mud in coastal environments

317

can occur under a protected setting where destructive wave and coastal currents are weak.

318

This dominance of very fine sediments was interpreted to be a result of deposition under an

319

environment of lesser energy by Assefa (1981), who suggested these sediments were

320

deposited in restricted lagoons or coastal lakes. In addition, the presence of sedimentary

321

structures such as thin, parallel, wavy and discontinuous clay laminations, flame structures

13

322

and associated fining-upwards bedding indicate intertidal conditions (Potter et al., 2005;

323

Davis, 2012; Didu, 2013; Fagherazzi et al., 2013).

324 325

4.2.1.6. Facies S6 (shale-limestone-marl facies)

326 327

This lithofacies is similar to facies S5 with the exception that in this sequence shale and

328

mudstone beds are intercalated with thin beds of limestone and marl (Fig. 6B). The average

329

thickness of shale and limestone beds ranges from 1.5 to 2.5 m and from 0.2-1 m,

330

respectively. Intercalations between shale, marl and limestone show wavy and irregular

331

bedding along with rare mud cracked surfaces (Fig. 6C). This facies is observed only in the

332

Upper Mudrock Member and marks the gradational contact to the overlaying Antalo

333

Limestone Formation.

334 335

Irregular alternations of marine carbonates along with continental sediments such as

336

mudrocks can be interpreted as periodic or seasonal fluctuation of continental sediment flux

337

under a broad and topographically low coastal environment, hence the irregular and wavy

338

carbonate and mudstone intercalation (Assefa, 1981; Potter et al., 2005). Such facies grades

339

into a carbonate dominated sequence indicating increasing marine influence of the

340

transgressing Tethys Sea (Russo et al., 1994).

341 342

4.2.1.7. Facies S7 (shale and lenticular gypsum facies)

343 344

This lithofacies manifests only in the Upper Mudrock Member. It is dominated by thinly

345

bedded stacks of shale intruded by large, white to light pink displacive gypsum nodules (rose

346

gypsum) forming a bird’s beak morphology (Fig.6D). The average thickness of shale beds

14

347

ranges from 0.5 to 0.7 m and that of gypsum beds ranges from 0.2 to 0.4 m. Apart from shale,

348

the gypsum nodules are surrounded by a marl and dolomitic limestone matrix.

349 350

Displacive growth of lenticular and nodular anhydrite within a clay and mud substrate is

351

typical in semi-arid and humid regions (Cody, 1979; Warren, 2016). Gypsum precipitation is

352

facilitated by seepage influx of marine water in supratidal mudflats (Warren, 2006, 2016).

353 354

[Insert Fig. 6 here]

355 356

4.2.2. Evaporite facies

357 358

4.2.2.1. Facies E1 (laminated gypsum facies)

359 360

This lithofacies is formed predominantly from laminated gypsum. Thickness of individual

361

laminated beds ranges from 0.5 to 2 m. laminated gypsum beds form multi-storey composite

362

beds with thickness reaching ~10 m. The composite beds are separated by 0.2 to 0.5 m thick

363

calcareous and gypsiferous shales and dolostones. Individual laminations can be traced

364

laterally for a considerable distance. In the Gohatsion locality, massive laminated gypsum

365

beds showing shoaling upwards facies are common (Fig.7A). Thinner (0.2 m) beds also show

366

gypsum pseudomorphs after bottom grown crystals. This lithofacies forms an erosive and

367

truncated contact with lithofacies E2 and E3 while its contacts with E6 and S4 are sharp (Fig.

368

7B). Laminated gypsum layers tend to form tepee structures (pressure ridges) in many

369

horizons (Fig. 7C). Some gypsum beds outcropping in the Gohatsion and Dejen sections

370

show nodules with preferred alignment and lenticular crystal structure (probably a result of

371

displacive growth). Lenticular gypsum crystals are often surrounded by wavy laminations of

15

372

gypsified microbial mats (Fig. 7D). At microscopic scale, the major texture observed is fine

373

grained alabastrine gypsum forming a xenotopic mosaic. Xenotopic anhydrite crystals also

374

form partly oriented to wavy fabric in some samples. Displacive gypsum crystals also occur

375

in anhydrite matrix. Microcrystalline subhedral to euhedral crystals along with trace amounts

376

of quartz, clay and opaque minerals can be found filling secondary porosity. Additionally,

377

some samples show the presence of thin irregular laminations of algae and microcrystalline

378

dolomite.

379 380

Laminated gypsum facies is common in shallow to deep subaqueous ponds and evaporite

381

lagoons (Warren, 2006; Babel, 2007; Matano, 2007; Schreiber et al., 2007). Increasing

382

salinity and brine mixing are favorable for the formation of bottom nucleation, justified by

383

pseudomorphs of gypsum crystals forming thin layers (Warren and Kendall, 1985; Babel,

384

2007; Hovorka et al., 2007). Thickness of individual layers also decreases as evaporation

385

intensifies and brine depth decreases (Warren, 2006). Truncated and sharp contacts between

386

laminations (layers) can be interpreted either as effects of seasonal subaerial exposure and

387

evaporite drawdown (Babel, 2007) or as seasonal flooding by under-saturated marine or fresh

388

water (Havorka et al., 2007). Warren and Kendall (1985) and Warren (2006) noted the

389

presence of tepee structures (or pressure ridges) in sabkha settings. The presence of gypsified

390

microbial mats and laminated gypsum interlayered with fine siliciclastic rocks can also be

391

regarded as evidence for prolonged periods of lowered brine salinity and water level

392

fluctuation (Matano, 2007).

393 394

[Insert Fig. 7 here]

395 396

4.2.2.2. Facies E2 (nodular gypsum facies)

16

397 398

This is the most dominant gypsum facies in all localities. It is composed of nodular,

399

enterolithic and chicken wire gypsum and gypsum mush, with algal laminations or

400

dolomudstone matrix (Fig. 8A, B and C). Calcareous mudstone and siltstone laminations are

401

also found interlaminated with gypsum beds. In the Gohatsion, Dejen and Jemma localities,

402

nodular gypsum facies forms multi-storey composite beds reaching up to 15 to 20 m in

403

thickness. Similar to lithofacies E1, the composite beds are separated by a 0.5 to 1.2 m thick

404

gypsiferous shale and dolomitic mudstone beds. This lithofacies usually has sharp and

405

erosive contacts with E1, E3 and E4 and a gradational contact with lithofacies S4. The size of

406

nodules varies, but in most cases, it ranges from pebble to cobble size. Thick alabastrine beds

407

with very coarse botryoidal alabastrine mosaic are also found mingled with nodular gypsum

408

beds. Locally, this facies shares both gradational and sharp contacts with laminated gypsum

409

beds (e.g., Fig. 7B). Intercalations with siliciclastic and marl beds show tidal cycles with

410

shallowing upwards facies (Fig. 8D and E). The size of nodules found in some beds in the

411

Dejen section, progressively decreases towards the top. Two dominant fabrics are identified

412

at microscopic scale. The first is a microcrystalline, xenotopic anhydrite matrix intruded by

413

displacive granoblastic secondary gypsum. The second fabric is represented by replacive

414

gypsum nodules surrounded by algae and dolomudstone (Fig. 8F). Granoblastic gypsum

415

nodules have also leached grain boundaries and relic anhydrite crystals floating within

416

gypsum grains.

417 418

In most case studies, formation of nodular alabastrine gypsum texture is associated with

419

diagenetic and telogenetic processes (e.g., Gindre-Chanu et al., 2015). Such textures are

420

mistaken for ‘primary’ nodular anhydrite formed in sabkha environment. Alternatively,

421

Warren and Kendall (1985) interpreted the presence of coarse alabastrine gypsum nodules

17

422

within thin algal lamella matrix along with lenticular gypsum nodules as typical formation of

423

primary anhydrite in sabkha environments (e.g., Fig. 8A). According to Warren (2006),

424

primary anhydrite growth in supratidal (sabkha) settings occur in the midst of microbial and

425

sediment deposition as a result of capillary evaporation. Additionally, this lithofacies is

426

closely associated with evaporitic dolomites and gypsum mush along with gypsified

427

microbial mat layers, which are a typical succession in a sabkha environment (Warren and

428

Kendall, 1985; Warren, 2006, 2016). Furthermore, the presence of carbonates interlayered

429

with gypsum indicates periodic refreshment of the brine, causing sporadic under-saturation

430

with respect to gypsum (Stefano et al., 2010).

431 432

[Insert Fig. 8 here]

433 434

4.2.2.3. Facies E3 (clastic gypsum)

435 436

This lithofacies is observed only in the Mugher, Dejen and Gohatsion localities. It is

437

composed of clastic gypsum layers formed from gypsrudites and gypsarenites. Gypsarenite

438

layers found in the Gohatsion locality are interlayered with thin and wavy mudstone

439

laminations with displacive gypsum crystals (Fig. 9A). Individual clasts are sub-rounded to

440

well-rounded, oval shaped and horizontally oriented, and often form thin beds, ranging from

441

0.5-1 m in thickness (Fig. 9B). In the Dejen and Mugher sections, however, clastic gypsum

442

layers tend to be formed from gypsrudites that also incorporate large pebble to cobble-sized

443

limestone and siltstone clasts (Fig. 9C).

444 445

Clastic gypsum facies may form under high energy conditions in shallow brine pools where

446

the wave base can affect the brine column, leading to the precipitation of very fine sand-sized

18

447

gypsum crystals (Havorka et al., 2007). In addition, clastic (detrital) gypsum and other

448

evaporites can form under in situ dissolution and karstification (Alberto et al., 2007). The

449

presence of post-depositional slumps near clastic beds along with secondary displacive

450

anhydrite nodules, associated with clastic gypsum beds in the Gohatsion and Dejen sections,

451

can be considered as proofs for karstification and groundwater influence, possibly during

452

telogenesis.

453 454

[Insert Fig. 9 here]

455 456

4.2.2.4. Facies E4 (heterolithic gypsum)

457 458

This gypsum facies consists of several types of lithologies, including silt, sand, clay and

459

carbonate laminae encrusted by gypsum. The term ‘heterolithic bedding’ is commonly used

460

to describe intercalations between sand and mud laminae (Potter et al., 2005) but in this

461

context, it is used to imply inter-lamination between siliciclastic sediments and evaporites

462

(e.g., Fig. 7C). This facies manifests itself in the Gohatsion, Dejen and Mugher sections,

463

where 0.1-0.4 m thick composite laminations of the above-mentioned lithologies are exposed.

464

The contact with facies E1 is gradational while the contacts with lithofacies E2 and E3 are

465

sharp. Apart from the heterolithic bedding style, wavy, discontinues and planar laminations

466

are also common.

467 468

Inter-bedded siliciclastic and evaporite rocks are indirect indicators of fluctuating water

469

depths in a brine pool (Hovorka et al., 2007). According to Warren (2016), seasonal flooding

470

and sediment influx may show mud drapes and layers with no obvious capillary textures in a

471

subaqueous setting. In such cases, the existence of syn-depositional textures such as wavy

19

472

beds may also indicate reworking caused by wave action. Accumulation of insoluble

473

sediments may form irregular laminations in dissolution cavities of gypsum beds during

474

rehydration of late diagenesis (Gindre-Chanu et al., 2015). Such a process usually leaves thin

475

irregular laminae at the base of re-precipitated gypsum layers.

476 477

4.2.2.5. Facies E5 (massive gypsum)

478 479

This facies is rare in the logged sections. Where identified, it is represented by massive

480

gypsum beds with 0.5-1.5 m thickness. This facies shows no significant texture or internal

481

bedding with the exception of the presence of alabastrine nodules noted in the Gohatsion and

482

Mugher sections.

483 484

Although the origin is not well-known, massive gypsum or anhydrite presumably represents

485

uniform conditions of deposition (Boggs, 2009). It is also suggested that massive gypsum

486

(rehydrated anhydrite) forms from a brine solution with salinity close to halite precipitation

487

conditions (Boggs, 2009 and references therein). The presence of alabastrine nodules and

488

displacive gypsum crystals can be the effect of burial diagenesis and subsequent uplift and

489

rehydration (Gindre-Chanu et al., 2015; Warren, 2016).

490 491

4.2.2.6. Facies E6 (carbonate facies)

492 493

This facies, unlike the previous evaporite facies, is predominantly composed of carbonate

494

rocks (dolomite and dolomitic limestone). Average thickness of beds range from 0.5 to 4 m.

495

This lithofacies marks the upper and lower gradational transition from the Gypsum Member

496

to the Upper and Lower Mudrock Members. It also shows a gradational contact with major

20

497

evaporite lithofacies such as E1, E2, E3, and E4. Major sedimentary features observed in this

498

facies are vuggy dolomite beds, fenestrae, microbial laminations and stromatolitic type

499

mounds (Fig. 10A and B), as well as small- and large-scale mud cracks. Carbonate beds are

500

dominantly dolomudstone with highly fossiliferous packstone pockets composed of

501

fragmented bivalve shells (Fig. 10C). Thin clay laminations also occur between carbonate

502

beds. In the Dejen locality, pisolitic layers and lenses associated with microbial mounds have

503

been observed (Fig. 10A). In most outcrops, the carbonates are dissected by tertiary satin spar

504

gypsum and displacive nodular anhydrite. Three microfacies have been recognized

505

microscopically: (i) dolomudstone composed of fine dolomitic crystals with vuggy fabric and

506

floating displacive gypsum crystals (Fig. 9D and E); (ii) dolowackestone with completely

507

dolomitized molds of gastropods (Fig. 9F); and (iii) dolomitic packstone with abundant

508

peloids and bioclasts and rare intraclasts (Fig. 9G). Localized bioturbations are also common

509

in the dolomitic packstone. All analyzed samples are highly oxidized. Syn-depositional

510

diagenetic textures such as isopachous cement, drusy calcite cement and both replacive and

511

displacive gypsum crystals are common (Fig. 10 D to G).

512 513

The presence of bioclastic pockets and associated bioturbations, dolomitic packstone and

514

dolowackestone horizons indicates subtidal origin (Matter, 1967). On the other hand, the

515

presence of mud cracks, microbial mounds and associated pisolites indicate an intertidal

516

setting (Matter, 1967; Warren, 2006; Rahimpour-Bonab et al., 2010).

517 518

Carbonate diagenesis can take place at different locations and stages of formation of the

519

rocks, ranging from near surface and marine diagenesis to meteoric diagenesis and at deep

520

burial environments (Freeman, 1997; Tucker, 2001). Marine diagenesis takes place in

521

shallow and deep marine settings along with intertidal to supratidal zones. This process is

21

522

marked by the presence of dissolution cavities, isopachous cement, and micritization along

523

with anhydrite cementation (Tucker, 2001; Rahimpour-Bonab et al., 2010), which are evident

524

in the analyzed carbonate samples in this study. Furthermore, Rahimpour-Bonab et al. (2010)

525

considered the presence of gypsum nodules and cavity fills as indicative of hypersaline

526

diagenesis under supratidal conditions. Meteoric diagenesis is associated with subaerial

527

exposure during sea level lowstand in lagoonal, intertidal settings. Since calcite is more

528

reactive than dolomite in such conditions, dissolution cavities, drusy and blocky calcite

529

cementation and calcite neomorphism is very common (Fig. 9A; Rahimpour-Bonab et al.,

530

2010).

531 532

[Insert Fig. 10 here]

533 534

4.2.2.7. Facies E7 (tertiary gypsum facies)

535 536

This facies represents secondary and tertiary gypsum found in the form of satin spar veins,

537

displacive gypsum crystals, fracture and vesicle filling alabastrine nodules affecting almost

538

all the lithologies found in the Gypsum Member of the studied localities (e.g., Fig. 8B and D;

539

Fig. 9C). At microscopic scale, the samples show the presence of displacive daisy gypsum

540

crystals and fracture veins filled by secondary subhedral selenite and secondary satin spar

541

gypsum.

542 543

Fibrous gypsum (satin spar gypsum) grows in hypersaline brine filled veins induced by

544

hydraulic fracture during various stages of exhumation (Warren, 2006; Matano, 2007;

545

Tucker, 2011). Voids and fractures filled with satin spar veins are indicative of the end of

546

burial diagenesis and beginning of telogenesis, i.e., diagenesis induced by exhumation

22

547

(Warren, 2006). Displacive selenite rosette gypsum crystals form when a series of near

548

equilibrium hydration phases exist during uplift and hydration. The slow rate of reaction

549

allows the formation of large selenite crystals in isolated nucleation sites. This commonly

550

happens when the gypsum horizon passes through a stagnant aquifer during uplift

551

(telogenesis) (Gindre-Chanu et al., 2010; Warren, 2016).

552 553

5. Discussion

554 555

5.1. Stratigraphic relationships

556 557

The Gohatsion Formation is described as having a ‘cyclic’ intercalation between fine-grained

558

siliciclastic and evaporite units, leading to several classification schemes of this formation

559

(e.g., Assefa, 1981; Russo et al. 1994; Gani et al., 2008; Dawit, 2010; Wolela, 2014). The

560

classification scheme by Assefa (1981) that assigned four informal members, namely the

561

mudrock member, the lower claystone member, the Gypsum Member and the Upper

562

Claystone Member was not adopted in this study primarily due to: (i) the “Mudrock Member”

563

of Assefa (1981) was not recognized in the logged and described sections of the current study

564

(Fig. 4) outside the type locality; and (ii) detailed field logging shows the dominance of

565

mudstones, shales and siltstones over clay stones in the upper and lower claystone members

566

of Assefa (1981). Hence, the name “Upper and Lower Claystone Member” is not a proper

567

representation of the lithostratigraphic succession. Therefore, a new classification scheme is

568

proposed whereby: (i) the Upper Claystone Member of Assefa (1981) is substituted by Upper

569

Mudrock Member; (ii) the Gypsum Member remains the same and (iii) the Lower Claystone

570

Member and the Mudrock Member of Assefa (1981) are combined and renamed the Lower

23

571

Mudrock Member. Lateral continuity of these informal members in the studied localities is

572

confirmed (see the geological maps in Fig. 3).

573 574

5.2. Depositional environment

575 576

A peritidal depositional setting is a distinct combination of sedimentary facies cycles

577

reflecting various phases of tidal sedimentation (Lasemi et al., 2010). In such environments,

578

subtidal, intertidal and supratidal signatures of diurnal or annual sedimentation form

579

cyclically (Davis, 2010). Assefa (1981) and Russo et al. (1994) considered the Gohatsion

580

Formation to be formed in a peritidal environment. However, these works did not fully

581

describe the distribution and pattern of these cycles. In the current work, the peritidal cycles

582

of the Gohatsion Formation with respect to the three informal members are fully described

583

(see summary in Fig. 4) and the depositional scenario for the three members are discussed.

584 585

5.2.1. The Lower Mudrock Member

586 587

In all measured sections and localities, this member has a sharp contact with the underlying

588

siliciclastic deltaic to barrier/spit inlet deposit [unit II of the Adigrat Sandstone in Dawit,

589

2010; and Dawit and Bussert, 2009]. The lithofacies assemblage of this member indicates a

590

tide dominated back barrier mudflat depositional system (Fig.11). Fluvial influence seems to

591

come from the northwestern side, justified by the occurrence of bioturbated and laterally

592

accreted channel scours in silty mudrock beds exposed in the Dejen and Jemma localities (see

593

Fig. 5B). In the Adigrat Sandstone Formation, the upper beds show paleo-current directions

594

dipping towards southeast, which are also concordant with the direction of marine flooding

595

(Dawit, 2010). In this scenario the facies stacking patterns such as shoreward decrease in

24

596

grainsize from muddy sand to sandy mud, wave dominated ripples, bioturbation and

597

heterolithic bedding closely resemble a transgressive facies model for tidal flats as proposed

598

by Flemming (2010). Preservation of the above-mentioned tidal signatures suggests rapid

599

sedimentation rates (Davis, 2010). The presence of evaporite beds such as gypsum and

600

dolomitic limestone towards the top of all the measured sections indicates a gradational

601

change in climate that favors the formation of evaporite rocks.

602 603

[Insert Fig. 11 here]

604 605

5.2.2. The Gypsum Member

606 607

The close relationship between gypsum textures and the hydrography has recently come into

608

the focus of scientific research (e.g., Babel, 2007). Aside from the localized variability of

609

textures, the overall texture is uniform throughout the sections with the majority of textures

610

falling under laminated or nodular types. These are some of the key characteristics of

611

evaporites (in this case gypsum), formed within tectonically inactive settings such as passive

612

margin basins (Schreiber et al., 2007). In the Gohatsion section, laminated gypsum facies

613

show typical stratified facies of deeper subaqueous basins. In this section, gypsum beds form

614

cyclic thinning upwards pattern which is indicative of increasing salinity (Warren, 2006). In

615

ancient evaporite platforms, shoaling upwards facies can be identified by a facies association

616

of poorly bedded/massive gypsum and carbonate mush, capped by bedded nodular gypsum

617

with preferential elongation and gypsum/anhydrite pseudomorphs after bottom grown

618

selenite crystals (Warren and Kendall, 1985; Fig. 7A). The presence of wavy beds of gypsum

619

(see Fig. 7D), thin bioclastic limestone beds (see Fig. 10C) and bioturbated dolomitic

620

packstone beds (see Fig. 10G) also indicates a shallow subaqueous system prone to

25

621

turbulence/storm (Boggs, 2009; Warren, 2016). Subtidal facies is represented by cyclic

622

interlayering of a shoaling upwards sequence with bioturbated dolomitic packstone and fine

623

calcareous mudstone and gypsiferous shale. The dominance of microbial mats and microbial

624

gypsum forming tepee structures, heterolithic gypsum, fenestral dolomitic mudstone and

625

packstone along with microbial mounds and pisolitic layers imply deposition within the

626

intertidal zone (e.g., Warren, 2006, 2016). Dominance of nodular gypsum interlayered with

627

gypsum and carbonate mush in siliciclastic sediment matrix typify the supratidal zone

628

(Warren and Kendall, 1985; Tucker, 2011; Warren, 2016). A sharp or an erosive contact

629

between nodular and laminated gypsum beds (Fig. 7B), reflects the presence of holomictic

630

brine, which allowed brine mixing and periodic refreshment from both continental and

631

marine water (Babel, 2007). Cyclic repetitions of the above-mentioned facies can be seen in

632

several quarry sites in all the studied sections (e.g., Fig. 8D and E). This stacking pattern also

633

resembles a typical sabkha succession along broad rimmed shelves in times of arid climates

634

(Tucker, 2011; Warren, 2016). The general facies stacking pattern and depositional model of

635

this member is summarized in Figure 12, and can be broadly grouped into a marine coastal

636

sabkha setting having a subtidal, narrow intertidal and broad supratidal zones.

637 638

Lithofacies associations in the measured sections indicate the dominance of subaerial

639

(sabkha) lithofacies in marginal areas (Dejen, Mugher and Jemma) whereas a subaqueous

640

facies dominates the rift area (Gohatsion). This suggests that marine influence was greater in

641

the Gohatsion section while continental influence was more prominent in the others.

642 643

[Insert Fig. 12 here]

644 645

5.2.3. The Upper Mudrock Member

26

646 647

In contrast to the Lower Mudrock Member, this member is composed of fine-grained

648

siliciclasts and carbonates. This dominance of very fine sediments was interpreted to be a

649

result of deposition under an environment of lesser energy (Assefa, 1981). The presence of

650

sedimentary structures such as thin uniform lamination as a result of suspension settling (see

651

Fig. 6A) strengthens this interpretation. Apart from background sedimentation structures,

652

localized tidal beds such as heterolithic and wavy laminations are common in some horizons.

653

Although tidal bedding could occur in a non-tidal depositional conditions (Didu, 2013), the

654

presence of associated fining-upwards bedding and micro-mud cracks in thinly bedded

655

carbonate rocks (Fig. 6C) may be regarded as indicative of deposition within the intertidal

656

zone. The evaporitic carbonate and gypsum rocks at the bottom suggest a supratidal setting,

657

while the massive shale and mudstone beds with no sedimentary structures in the middle,

658

along with the presence of tidal facies, indicate a broad coastal mud flat. The absence of salt

659

marsh-type facies may indicate that the mudflat was non-vegetated. At the top, interlayering

660

of partly fossiliferous limestone and marl indicates seasonal sea level rise and flooding of the

661

basin by marine water (Assefa, 1981). Interlayering of carbonate and fine siliciclastic rocks is

662

common in low relief and broad shelf settings (Potter et al., 2005). This suggests that this

663

member was deposited under a low energy broad shelf created by the southeast transgressing

664

sea (Fig. 13). Furthermore, Russo et al. (1994) noted the presence of gastropods and bivalves

665

(e.g., Arcomytilus) and gave an interpretation of mixed marine setting with fresh water

666

influences from adjacent areas.

667 668

[Insert Fig. 13 here]

669 670

5.3. Regional paleogeographic and paleoclimatic implications

27

671 672

The early to middle Jurassic represented a critical time for the development of a passive

673

margin around the newly disintegrated East African and Madagascar blocks (Bosellini 1992;

674

Scotese et al. 1999; Schandelmeier et al., 2004; Mette, 2004; Golonka, 2007; Dawit 2010).

675

The rift to drift transition of east and west Gondwana facilitated the transgression of the

676

Neotethys Sea. As a result, the Somalian-Ethiopian-Madagascan Gulf was formed

677

(Schandelmeier and Rynolds, 1997; Bosellini 1992; Scotese et al. 1999; Golonka, 2007).

678 679

In the current boundaries of Ethiopia, and the surrounding region, marine transgression

680

started in the southeast, forming an epeiric sea (Assefa, 1981; Russo et al., 1994; Geleta,

681

1997; Dawit, 2010). The Gohatsion Formation in the Blue Nile Basin, the Hamanlei

682

Formation in the Ogaden Basin and Unit III of the Adigrat Sandstone Formation represent the

683

initiation of this transgressive phase (see Fig. 2; Assefa, 1981; Russo et al., 1994; Geleta,

684

1997; Hunegnaw et al., 1998; Dawit, 2010). Even though the depositional facies given to

685

these units are tidal flat and an associated restricted lagoon environment, there exists a

686

systematic change in terms of sediment type and mode of formation. In the Ogaden Basin,

687

formation of carbonate platform-rimmed sabkha-type deposits with little to no clastic

688

influence typify the Hamanlei/Iscia Baidoa Formation (Bosellini, 1992; Geleta, 1997;

689

Hunegnaw et al., 1998; Worku and Astin, 1992). In the Blue Nile Basin, mixed clastic and

690

evaporite deposition occurred in a tidal flat and lagoon environment, whereas in the Mekele

691

basin, deposition was completely dominated by the clastic rocks of Unit III of the Adigrat

692

Sandstone Formation (Dawit, 2010). In this scenario, the Gohatsion Formation represents a

693

transitional change in sediment type and supply from carbonate dominated to clastic

694

dominated shelf.

695

28

696

The global climate system shifted from a mega-monsoonal system to cool/dry sub-tropical to

697

a humid climate during the middle Jurassic (Scotese et al., 1999; Ziegler et al., 1983;Golonka

698

and Ford, 2000; Dawit, 2016). At this time, the paleogeography of East Africa was between

699

the latitudes of 15º to 25º South (Schandelmeier and Rynolds, 1997). This configuration puts

700

the palaeoclimate of the region in a dry sub-tropical to humid tropical climate (Scotese et al.,

701

1999; Golonka and Ford, 2000). This climate agrees with the formation of the

702

evaporite/carbonate/siliciclastic deposits of the Gohatsion Formation.

703 704

6. Conclusions

705 706

Detailed field observations and logging of four stratigraphic sections accompanied by

707

petrographic analysis provided a new insight into the depositional history of the Gohatsion

708

Formation. The main conclusions of this study are:

709 710

1. The informal classification of the Gohatsion Formation by Assefa (1981) into four

711

members has been revised based on new data from detailed field investigation of the

712

Formation in four localities distributed within the basin. Accordingly, the formation is

713

divided into three members: the Upper Mudrock Member, Gypsum Member and

714

Lower Mudrock Member.

715

2. Lithofacies associations of the Lower Mudrock Member show a typical siliciclastic

716

back-barrier tidal flat facies with a progressive grain size increase towards southeast,

717

which corresponds with the direction of the transgressive sea.

718

3. Lithofacies of the Gypsum Member shows a marine coastal sabkha setting.

719

Lithofacies distribution in the measured sections shows systemic variations, indicative

720

of subtidal, intertidal and supratidal cycles. Subaerial facies dominates marginal areas

29

721

(Mugher and Jemma) whereas subaqueous facies is dominant in the rift (Gohatsion);

722

hence, marine influence was grater in the rift while continental influence was greater

723

towards the marginal areas.

724

4. Lithofacies of the Upper Mudrock Member shows a gradational facies change from

725

supratidal to a broad siliciclastic/carbonate shelf and a localized intertidal flat setting

726

that eventually evolved to the overlying marine facies.

727

5. The Gohatsion Formation in the Blue Nile Basin should be regarded as a marker

728

formation for the basin evolution from a continental to a transgressive marine setting

729

during the middle Jurassic.

730

6. Lithofacies analysis is in agreement with the established paleogeography and

731

paleoclimate of the region which favors deposition under a broad, tide-dominated

732

coast and dry sub-tropical to humid tropical climate.

733 734

Acknowledgments

735 736

This work has been conducted as part of MSc thesis of SGC. The Addis Ababa University is

737

acknowledged for partly funding the work. Million Alemayehu, Bahru Zinaye and Takele

738

Mengistie are duly acknowledged for their assistance in the field. Special thanks is due to

739

Prof. Anneleen Foubert for her critical comments regarding the sedimentology. We would

740

also like to thank Nils Lenhardt and Nahid Gani for their rigorous reviews and critical

741

comments that benefited the manuscript immensely.

742 743

References

744 745

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Matter, A., 1967. Tidal flat deposits in the Ordovician of western Maryland. Journal of Sedimentary Petrology 37, 601-609. Pik, R., Daniel, C., Coulon, C., Yirgu, G., Hofmann, C., Ayalew, D., 1998. The northwestern

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Sonyea Group of New York. Journal of Sedimentary Research 69, 909-925.

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crisis. Palaeogeography, Palaeoclimatology, Palaeoecology 297, 83-99.

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note on geology and mineral map of Ethiopia. Journal of African Earth Sciences 36,

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Warren, J.K., 2006. Evaporites: sediments, resources and hydrocarbons, Springer Verlag,

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Berlin Heidelberg, Germany, 1041p. Warren, J.K., 2016. Evaporites: a geological compendium, second ed., Springer International Publishing, Switzerland, 1822p.

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Warren, J.K., Kendall, C.G. ST.C. 1985. Comparison formed in Marine Sabkha (subaerial)

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and Salina (Subaqueous) settings-modern and ancient. The American Association of

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Petroleum Geologists Bulletin 69(6), 1013-1023.

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Wolela, A., 2008. Sedimentation of the Triassic-Jurassic Adigrat Sandstone Formation, Blue

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Nile (Abay) Basin, Ethiopia. Journal of African Earth Sciences.

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Doi:10.1016/jafrsci.2008.04.001

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Wolela, A., 2009. Sedimentation and depositional environments of the Barremian-

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Cenomanian Debre Libanos Sandstone, Blue Nile (Abay) Basin, Ethiopia. Cretaceous

900

Research 30, 1133-1145.

901

Wolela, A., 2014. Diagenetic contrast of sandstones in hydrocarbon prospective Mesozoic rift

902

basins (Ethiopia, UK, USA). Journal of African Earth Sciences 99, 529-553.

903

Worku, T., Astin, T.R., 1992. The Karroo sediments (Late Paleozoic to early Jurassic) of

904 905

Ogaden Basin, Ethiopia. Sedimentary Geology 76, 7–21. Ziegler, A.M., Scotese, C.R., Barrett, S.F., 1983. Mesozoic and Cenozoic paleogeographic

906

maps. In: Brosche, P., Sundermann, J. (Eds.), Tidal Friction and the Earth’s Rotation.

907

Springer Verlag, Berlin, pp. 240-252.

908 909

Figure Captions

910 911

Figure 1. (A) Paleogeography of Gondwana during late Paleozoic to middle

912

Mesozoicshowing the distribution of the Karoo rift (detail of the area marked by

913

the open triangle is given in Fig. 1B) (afterZiegler et al., 1983; Scotese et al., 1999;

914

Golonka and Ford, 2000;Schandelmeier et al., 2004;Catuneanu et al., 2005;Dawit,

915

2014); (B)Tectonic map of eastern Africa showing various intracontinental rifts

916

developed since the Paleozoic(after Bosworth, 1992; Bosellini, 1992;Gani et al.,

37

917

2008;Wolela, 2014); (C) Geological map of the Blue Nile Basin(after Jepsen and

918

Athearn, 1961; Dawit, 2010, 2016);lettered boxes represent the geological maps in

919

Figure 3.

920 921 922

Figure 2. Chrono-litho-stratigraphy of the Ogaden, Blue Nile and Mekele basins (after Geleta, 1997; Wolela, 2008, 2009, 2014; Dawit, 2014, 2016). Figure 3. Geological map of the studied localities where detailed sections and samples were

923

collected (see Fig. 1C and Fig. 4): (A) Mugher locality;(B) Gohatsion and Dejen

924

localities (boxes B and C in Fig. 1C); and (C) Jemma locality (box D in Fig. 1C).

925

Figure 4. Lithostratigraphic sections of the studied localities: (A) Mugher locality;(B)

926

Gohatsion locality; (C) Dejen locality; and (D)Jemma locality. Inset shows the

927

areal extent of the outcrops of the Gohatsion Formation within the BNB and the

928

locations of the measured sections.

929

Figure 5. Field photographs showing lithofacies of the Lower Mudrock Member: (A)

930

heterolithic bed composed of siltstone and mudstone from Dejen locality; (B)

931

migrating channel scours filled by siltstone indicated by the white dashed lines,

932

form Dejen locality;(C) lenticular siltstone beds within shale from Dejen locality

933

(pen is 15cm long); (D) micaceous siltstone bed also showing bioturbation (Bio)

934

from Gohatsion locality; (E) an outcrop of sandstone from Gohatsion locality

935

showing lithofacies S3 (geological hammer is 30cm long) and (F) photomicrograph

936

of sandstone (sample from the sections indicated on Fig.5E) showing well sorted

937

and rounded quartz arenite with calcite cement (Cal) and rare rock fragments (Rf).

938

Figure 6. Field photographs showing lithofacies of the Upper Mudrock Member: (A) thin

939

laminations formed by laminated silty (Slt) and clay (Cly) layers within mudrock as

940

a result of suspension settling; (B) an outcrop showing a gradational contact

941

between the Upper Mudrock Member and the Antalo Limestone Formation,

38

942

Gohatsion locality; (C) marl unit showing mud crack;and (D) intercalation of rose

943

gypsum with shale, Gohatsion locality.

944

Figure 7.Field photographs showing lithofacies of the Gypsum Member:(A) shoaling

945

upwards (thinning upwards) facies in gypsum; magnified view showing

946

pseudomorphs of gypsum after bottom grown selenite crystals (bgyp), quarry

947

outcrop in Gohatsion locality near Filiklik town;(B) an outcrop from Mugher

948

locality showing an erosive contact between microbial laminated gypsum (Lam)

949

and nodular chicken wire gypsum (Nod) indicative of change from intertidal to

950

supratidal environment (geological hammer 30cm long); (C) an outcrop from

951

Gohatsion locality showing heterolithic gypsum facies with thin carbonate and

952

calcareous silt and gypsified microbial mat;thinly dashed yellow lines indicate

953

tepee structures and white arrows indicate soft sediment deformation; and

954

(D)laminated gypsum layer showing wavy beds hosting displacive lenticular

955

gypsum crystals indicated by white arrows, from near Filiklik town, Gohatsion

956

locality (marker pen is 14 cm long).

957

Figure 8.(A)Photomicrograph of nodular gypsum from Dejen locality showing replacive

958

gypsum crystal (Gyp) with anhydrite relic inclusions capped by algal filament

959

(Alg), indicative of primary anhydrite growth under sabkha conditions; (B) primary

960

and secondary gypsum nodules matrixed by dolomudstone and gypsum forming

961

thin beds in Dejen locality; the white arrows show secondary displacive gypsum

962

crystals (marker pen 14cm long); (C) gypsum mush layer with coarse-grained

963

alabastrine nodules (Alb) and enterolithic layers (ent) separated by thin gypsum

964

crest (gpcr) and thin calcareous sediments, Mugher locality;(D) section from the

965

Jemma locality showing an intercalation of gypsiferous shale deposited under

966

subtidal condition overlain by intertidal silty marl bed with mud cracks (Marl) and

39

967

intercalation of gypsiferous shale (GSh) and nodular gypsum (Nod) indicating

968

supratidal sequence; vertical and oblique veins filled by secondary satin spar

969

gypsum; (E) a quarry outcrop at the Gohatsion section showing sabkha cycles

970

indicated by the black arrows, where laminated gypsum layers and associated

971

lagoon mud and marl indicate subtidal deposition, while thinly laminated marl and

972

dolomitic limestone show intertidal deposition, capped by nodular gypsum

973

indicating supratidal origin; and (F) calcareous gypsum having displacive

974

xenotopic (xeno) and replacive granoblastic (gran) gypsum crystals crosscut by

975

tertiary satin spar gypsum (stsp) with microcrystalline dolomudstone matrix (Dol);

976

magnified picture at the bottom right corner indicates the presence of relic

977

anhydrite crystals and the black arrows indicate leached grain boundaries of the

978

granoblastic gypsum.

979

Figure 9. Field pictures of clastic gypsum facies: (A) thin layers of gypsarenites (gpar)

980

interlayered with mudstone (mud), the white arrows indicate secondary displacive

981

lenticular gypsum crystals, Gohatsion locality; (B)pebble size gypsum clasts

982

matrixed by gypsum, indicative of reworking, from near Filiklik town; and(C)

983

limestoneclasts trapped in gypsum matrix of clastic gypsum also showing

984

displacive telogenetic gypsum (Tel)(Tip of the hammer is 17cm long).

985

Figure 10. Field pictures and photomicrographs of carbonate facies: (A) cross-section of

986

microbial mound showing stromatolitic type lamination (Str);top view shown in

987

the left bottom corner; the mound overlays pisolitic layer (Pis), Dejen locality; (B)

988

vuggy dolomitic limestone also showing pisolites (Pis), Gohatsion locality; (C)

989

bioclastic limestone bed showing pockets of unidentified fossil fragments,

990

indicated by the white arrows, Gohatsion locality; (D) photomicrograph of

991

dolomudstone sample with scattered displacive gypsum crystals (Gyp) and

40

992

fenestric cavities (Por); (E) dolomudstone with vuggs filled by secondary selenitic

993

gypsum crystals and replacive granoblastic gypsum, indicative of supratidal

994

setting; (F) photomicrograph of dolowackestone with complete mold of

995

gastropod;the gastropod mold is composed of fine crystalline dolomite filling and

996

drusy calcite rim, indicative of active dissolution and replacement; and (G)

997

photomicrograph of dolomitic packstone showing several allochems of peloids

998

(Pel), bioclasts and bioturbations (Bio); early diagenetic textures such as

999

isopachous calcite cement (Iso) and drusy calcite cement can also be seen which

1000

are indicative of shallow, syn-depositional meteoric diagenesis.

1001

Figure 11.Depositional model for the Lower Mudrock Member: composite stratigraphy (A)

1002

and possible paleogeographic setting and the direction of transgressive sea (B);‘I'

1003

represents the Dejen, Mugher and partly Jemma localities, whereas ‘II’ represents

1004

Gohatsion and partly Jemma localities.

1005

Figure 12. Depositional model for the Gypsum Member, showing different textures and

1006

associated depositional environments of evaporite and associated siliciclastic rocks

1007

along with the possible paleogeographic setting of the transgressive sea.

1008

Figure 13. Depositional model for the Upper Mudrock Member: composite stratigraphy (A)

1009

and possible paleogeographic setting during deposition (B).

41

Research Highlights 1. 2. 3. 4.

The Gohatsion formation has been reclassified into 3 informal members. Depositional environment for the Lower Mudrock Member is mixed tidal flat Depositional environment of the Gypsum member is marine coastal sabkha Depositional environment of the Upper Mudrock Member is a broad coastal shelf

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: