p.1

UPPER CRUSTAL SUCCESSION IN WADI BIDI-HILTI AREA
Susumu Umino (Kanazawa University, JAMSTEC) and Yuki Kusano (Geological Survey of Japan)

Stratigraphy and Lithofacies of the Upper Crust
The Cretaceous Oman Ophiolite in the west of Sohar,rocks, “V3 unit” lavas (equivalent to “Salahi Unit” ofnorthern Oman Mountains, preserves the entire sequence of

Alabaster et al., 1982), are the latest volcanic productvolcanic rocks which formed the ophiolite. The volcanic

comprising the ophiolite, and were emplaced onto thick off-

sequence is largely divided into three stratigraphic units based

axial pelagic sediment covering erosional surfaces of the V2

on both field relationships and lava geochemistry (Lippard etflows and pyroclastic rocks (Bishimetal Exploration Co. Ltd.,

al., 1986; Bishimetal Exploration Co. Ltd., 1987a; 1987b;

1987b; Ernewein et al., 1988; Lippard et al., 1986; Umino et

Umino et al., 1990, 2003, Umino, 2012); Stage 1) The loweral., 1990, 2003, Umino, 2012). Radiolarian fossils in theeffusive rocks, “V1 unit”, formed on and close to thesedimentary rocks interbedded with and below V3 volcanic

paleospreading axis at 100-96 Ma, which are geneticallyunit give early-middle Turonian age (93.5-ca. 90 Ma)

linked to the layered gabbros and sheeted dike complex(Kurihara and Hara, 2012; Lippard et al., 1986; Tippit et al.,(Kusano et al., 2012).; Stage 2) The middle effusive rocks,1981). These sedimentary ages indicate that V3 unit was most“V2 unit” and related late intrusive rocks formed above an

likely erupted at or slightly after ~90 Ma, after a quiescentincipient subduction zone related to the initial detachment of

period indicated by the 30-50 m thick pelagic shale and before

the ophiolite at 96-94 Ma. The second stage lavas and dikes

the beginning of ophiolite obduction onto the Arabian

are tholeiitic and calc-alkalic rocks including high-Ca, low-

Continent at Campanian age (later than ~75 Ma) (Lippard etsilica boninite, and are readily distinguishable from theal., 1986; Robertson, 1987; Shervais, 2001).

aphyric V1 lavas and sheeted dikes with MORB-like major

Here we will visit the entire succession from the theand trace element characteristics (Umino et al., 1990, 2003;

uppermost V3 alkali flows through the V2 arc tholeiite to the

Einaudi et al., 2003; Ishikawa et al., 2002; Nagaishi, 2008;

V1 MORB extrusive rocks and the underlying sheeted dike

Kusano et al., 2014, 2017); Stage 3) The uppermost effusivecomplex exposed between Wadi Bidi and Wadi Hilti.

Fig. 1. Geological outline of northerm Oman
Ophiolite. Rectangles a and b are shown in Figs. and .

p.2

V3 large lava flow field

Fig. 2. Geological map of the V3 flow field. Lines A-A' to J-J' are thelocations of geological sections shown in Figure 4. Localities referred to in
the text are shown by crosses. Contours are altitude in meters.

p.3
The V3 unit comprises a compound lava flow field of

pahoehoe and pillow lobes that merge upstream into larger

alkali olivine basalt extruded onto and partially intruded into

and thicker sheet-like lava. Welding of flow lobes formed

thick pelagic sediment at or later than ca. 90 Ma (Umino,

domal structures of columnar joints with narrowing joint2012). The remaining flow field exceeds 1.2 km in volume,

spacing and increasing crystal number density toward thebut the original net volume is impossible to estimate due toupper boundary of domes and banded lava crust with erosive

the hidden and lost volumes. The V3 flow buried off-axial

coarse-grained lenticular dolerite embedded in fine-grained

fault-bounded basins with sporadic talus rubble at the foot of

convex dolerite (Fig. 4). Cooling rate determines thefault scarps and smooth basin floors consisting of the V2 flow

relationships among joint spacing, crystal number density andwith a thick sedimentary cover. The feeder dike for the V3

the degree of flow-lobe welding/coalescence. Higher supply

flow runs at a high angle to the paleospreading axis (Figs. 2,

rate of lava and slower cooling enabled flow lobes to merge3), which is expected for the emplacement under an off-axial

and subsequently inflate to form a larger sheet flow or toregional stress field. The feeder dike is as thick as 60 m near

become a part of the major lobe. On the contrary, relatively

the bottom of the exposed portion of the dike, 320 m below

faster cooling prohibits welded flow lobes to merge into an

the base of the V3 unit. The low length/thickness ratio and theinflating single larger lobe. The central facies of the V3 flow

skewed variation in thickness for the V3 dike is ascribed tofield consists of merged and inflated major flow lobes

the preferential emplacement at the apparent level of neutral

overlain and underlain by compound flows of pillow andbuoyancy formed in the shallow crust under thermal stress.

pahoehoe lobes as in the marginal facies.

3

Compound flows of the upper V3 flow field consist of

Fig. 3. Feeder dike of the V3 flow field. (a) The dike stands as a subvertical ridge (white arrows) intruded into thesurrounding V2 lava. Note a person as a scale (black arrow) (b) Folded wrinkles on the surface of the dike wall. Tightly
folded crust plunges upper right, indicating a southeastward flowage (white arrows) of magma within the dike. (c) Variationof the dike thickness with depth from the base of V3 unit. (d) Schematic section showing the V3 flow and its feeder dike.
Open arrows indicate the flow directions of magma inferred from the asymmetric wrinkles on the dike margins. The
exposed dike is most likely the thickest portion of the dike (dark hatched) emplaced at the apparent level of neutral
buoyancy. The dike may extend further below the surface but is considered to be much thinner than the exposed section
on the surface (light hatched).

p.4

Fig. 4. Structure of the main sheet lava of the central facies. (a) Simple structure of a sheet with a massive lava core
sandwiched by columnar jointed, thick upper crust and thin lower crust. (b) Complex structure of thick upper crust
consisting of multiple layers of columnar joints (arrows) intercalated with massive and banded lava. (c) Enlargement of
the rectangle in (b). Banding develops in the lower part of the upper crust, which is overlain by finely columnar jointed
layer. (d) Close-up view of banding, which shows soft and eroded, light coarser-grained dolerite lenses embedded in darkfiner-grained dolerite lava. Note the coarser dolerite lenses pinch and swell, and are partially connected together. Scale on
the right is 1 m long.

p.5
Fig. 5. Schematic model of emplacement of the V3 flow
field. (a) Lava extruded at low to moderate rates onto a
subhorizontal seafloor overlain by thick pelagic sediment.
Since water-saturated unconsolidated sediment could not
hold the dense lava, it intruded along the boundary
between the lower semi-consolidated and the upper
unconsolidated sediments. (b) Slowly advancing lava front
branched into numerous pahoehoe lobes, which coalesced
together to form a vast sheet-like lava and subsequently
inflated behind the flow front. (c) Some sedimentarylenses were uplifted with the inflating sheet and overlain
by the following paheohoe sheets, which then merged into
the main sheet of lava. (d) In addition to pahoehoe andpillow lobes, the marginal facies comprises coalesced but
unmerged lobes emplaced in the peripherals and above theinflating main sheet. Fast cooling rates prevented themfrom completely unifying with the main sheet.

Low-T hydrothermal alteration and weathering affected
LILE compositions of the V3 flow. However, strong positive
correlations among incompatible HFSEs and REEs, andrelatively good correlations with Zr show that these elements
were less mobile and preserve primary characteristics. V3
flow comprises trachybasalt to basaltic trachyandesite
dolerite with intermediate trace element characteristics
between OIB and E-type MORB. Whole-rock major and traceelement variations through a vertical transect at 8.7 km fromthe feeder dike show fractional crystallization of augite,
plagioclase and magnetite (Fig. 6). By contrast, other samplesof V3 flow show highly scattered whole-rock compositions,which may be explained by internal mixing of variably
differentiated magmas.
N-MORB normalized primitive V3 trace element
patterns show LREE enrichment in spite of similar HREE
abundances to N-MORB. Geochemical batch partial melting
model of depleted MORB mantle indicates that the primitive
V3 trace element compositions can be reproduced by themixing of melts formed by 0.2 wt% partial melting of garnet
lherzolite and 1.5 wt% to 3.0 wt% partial melting of spinellherzolite (Fig. 7).

Fig. 6. Stratigraphic variation in Yb at 6.7 (left) and 8.7 km (middle) from the source vent and TiO2 and MgO vs Yb for all V3
flows (right).

p.6

Fig. 7. N-MORB normalized trace element patterns for modeled primitive V3 lava. (a) Calculated trace elements patterns bybatch partial melting. Circles, squares and triangles denote 0.1 wt%, 0.2 wt% and 0.3 wt% melting of garnet lherzolite,
respectively. (b) Calculated trace elements patterns by batch partial melting. (c) Calculated trace element patterns by fractional
partial melting. See text for the detail.

p.7

V2 extrusive sequence in Wadi Bidi-Hilti area
The 1110-m-thick V2 sequence in Wadi Bidi-Hilti area

Narrow ranges of incompatible element variations and wideconsists of the 970-m-thick lower (LV2) and 140-m-thickvariations in compatible elements correlated with olivine

upper (UV2) subsequences (Fig. 8; Kusano et al., 2014). The

phenocryst contents suggest a small amount of olivine

LV2 occupies the majority of the V2 sequence. The lower

fractionation. The higher LaN/SmN and lower Hf/Ta ratios of

LV2 consists of pahoehoe flows and the upper part consists of

the UV2 indicate a more depleted source mantle and a larger

sheet flows. A few pillow flows occur at c. 650 m above thecontribution of slab-derived fluids for the UV2 than for thebase of the LV2 sequence (mab). A cylindrical magma conduit

LV2 magmas (Fig. 9).

5–7 m in diameter is preserved at 760 mab. Based on the

LV2 magma was formed by remelting of the V1 residue

occurrence of these lava flows with a small amount ofwith the addition of 1.7% amphibolite-derived fluid (Kusano

volcaniclastic rocks, the LV2 sequence in this area represents

et al., 2017). UV2 can be divided into low-Si UV2 and high-the proximal facies of a flat-topped submarine volcano. The

Si UV2 based on major and trace-element characteristics. The

LV2 samples have IAT characters based on their bulk rocklow-Si UV2 has a transitional composition between LV2 andchemistry, and can be divided into five geochemical cycles

high-Si UV2, and is distinguished from the high-Si UV2 byresulted from the variable degrees of fractionation of olivine

lower SiO2, and LILE, and higher Na2O, HFSE, and REE

and clinopyroxene (Fig. 9). The LV2 is terminated by a 1.0-

concentrations. The low-Si UV2magmas were generated bym-thick umber and overlain by boninitic UV2 consisting of

the introduction of high-temperature hydrous fluids and lesser

lobate sheet flows intercalated with several sedimentary

amounts of sedimentary melt (Fig. 10). In contrast, generationlenses and overlain by a 2 m thick pyroclastic fall deposit. Theof the high-Si UV2 magma required both hydrous fluid and

UV2

and

sedimentary melt, as indicated by the Hf and Nd isotope

clinopyroxene phenocrysts of length c. 3 mm. In addition togeochemistry. The high-temperature mantle wedge indicatedthese, the depleted and spoon-shaped REE patterns indicate

by the genetic conditions of the primary UV2 magma and thethat the UV2 lavas are high-Ca boninites, similar to thosehigh-temperature slab recorded by the Cpx amphibolite in thereported from Wadi Jizi (Fig. 10; Ishikawa et al., 2002).

metamorphic sole are consistent with a shallow and buoyant

rocks

generally

have

abundant

olivinesubduction zone, in which the absence of convection within
the mantle wedge caused progressive remelting and depletionof the source mantle wedge, resulting in the short-lived (<3m.y.) V2 arc magmatism (Umino et al., 2019).

Fig. 8. Columnar section of V2 sequence and simplified
geologic map of the Wadi Bidi-Hilti area (Kusano et al.,2014).

p.8

Fig. 9. Stratigraphic variationsof TiO2 and trace elements of theV2 sequence. Diamonds: LV2,
Crosses: UV2 (Kusano et al.,2014)

Fig. 10. Trace element patterns of V1 (a)normalized against primitive mantle,

LV2 (b) and UV2 (c) normalized against
N-MORB (Kusano et al., 2017)

p.9

Fig. 11. Geochemical modelling of LV2 and UV2 magmas by remelting of the residues of the V1 MORB and the V1 andLV2 residues, respectively (Kusano et al., 2017).

p.10

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p.12

Table 1. List of STOPs.
STOP

Long

Lat

Note

1

56.5670 24.1018 Entrance to Hilti Road; Lookout of Batina Olistostrome/V3/V2 extrusive unit

256.5582 24.1078 Overview of the V3 lava flow structure

356.5495 24.1096 V2 lava conduit and tumuli

456.5414 24.1159 V2/>0.5 m thick shale/V1

5

56.5230 24.1088 V1 fissure vent656.5123 24.1118

Sheeted dike complex

Localities of STOP-1 to -6. Qgw: Quaternary terrace deposit, Os: Olistostrome, SU2 & SU3: Umber, Sh: Metalliferous
sediment, SE3: V3, SE2: V2, SE1: V1, SD: Sheeted dike complex

p.13

STOP-1. Lookout of Batina Olistostrome over the V3-V2 extrusive succesion

Fig. 1. Overview of the upper part of the ophiolite. Stop-1 is on a hill in the Batinah Olistostrome (front). Behind theolistostrome are thick sheet flows and the feeder dike of V3 unit, which is underlain by V2 unit flows.

STOP-2. Overview of the V3 lava flow structure overlying the UV2 boninite flows and pyroclasts

Fig. 2. Basic structure of the central facies of V3 large lava flow,
showing the upper crust, core and lower crust (top and left).
The V3 flow field may be divided into central and marginal
facies on the basis of structure. Central facies comprises thick sheet
flows with a thickness more than 150 m, consisting of relatively
homogeneous, massive lava sandwiched between columnar jointed
lava. The simplest structure of the central facies consists of thickcolumnar jointed lava, massive lava and thin columnar jointed lava,
from top to bottom, which are hereafter referred to as upper crust, coreand lower crust (Fig. 2). Crust may be sparsely vesiculated especiallynear the flow top and bottom, while cores are virtually vesicle free. Lower crust is generally much thinner (~10 m) than upper
crust. Upper crust may include zones of domal domains composed of curved, radially arranged columnar joint, which at firstglance resemble to “entabulature” of subaerial flood basalt flows such as the Columbia River Basalt and Giant’s Causeway.
However, the majority of central facies shows more complex structures in that they have more than two sets of crust and core
within a single sheet of lava.
Lobate sheet flows and pyroclastic rocks below the V3 flow andexposed along the road are boninite of the UV2 sequence (Fig. 2).
Although the rocks are severely weathered, clinopyroxene
phenocrysts are still preserved and concentrated as green sand inwadis.

Fig. 3. Pyroclastic rocks of boninite in the UV2 exposed on the roadcut.

p.14

STOP-3. Conduit and tumuli of V2 lava

Fig. 8. Conduit (left) and tumulus (right) of the V2 extrusive unit
The conduit stands subvertically with a diameter of 5 X

is composed of embayed, lenses of coarser-grained basalt

7 m in the V2 lava flows and has concentric layers of fine-embedded in finer-grained lava. These are superposed,and coarse-grained lava. Such concentric layering is wellcoalesced pahoehoe lobes comprising the outer crust of aknow for lava tubes in Hawaii which form by quiet extrusion

tumulus, which were unable to infalate due to rapid cooling.

of lava for a long (>one week) period (Kauahikaua et al.,The lower part of the lava body is a completely merged flow1998), and is considered to be an equivalent sturcture of a lava

lobes that inflated to form the domal uplift of the upper crust

tube. The lava body on the right rests on a thin shale and haveof the tumulus.

lobate upper surfaces. The upper part of the lobate lava body

p.15

STOP-4. Conformable boundary between the V2 and V1 extrusive units
V2 extrusive unit shows either structural conformity andunconformity to the lower V1 extrusive unit. Where thegeneral trend of lava flows and interbedded sediments of theV2 unit is oblique to the structure of the underlying V1 flows,
volcanic conglomerate or tuff breccia comprising clasts
derived from the lower V1 lava is present on the eroded

Where structures of the V1 and V2 flows areconcordant, conformable relationship can be foundbetween the two extrusive units (Fig. 10). V2 sheet
flows conformably overlie a V1 massive lava with a

0.5-m thick reddish brown jasper bed between the
two unitssurface of the V1 unit and beneath the V2 unit (Fig. 9).
.

Fig. 9. Volcanic conglomerate on the dissected surface of the V1 (left of the photo) flows and overlain by the V2 flows(right)Fig. 10. Sheet flow of V2 unit is conformably overlying a thick massive lava of V1 unit at STOP-4

p.16

STOP-5. Fissure vent of V1

Fig. 11. Welded spatter and dense scoriae filling a fissure vent of V1 unit

Fig. 12. Bun-shaped and ameoboidal lava lobes of V1 unit
The V1 extrusive rocks form lobate sheet and pillow

Umino et al., 2000; 2002). Lava wrinkles are common on theflows intercalated with pyroclastic rocks, which commonly

surface of lobes. Some pahoehoe lobes can be traced upstream

occur as driblets and dense scoriae fed from a fissure vent (Fig.

to merge into thicker sheet-like lava, or are connected with

11). Lobate sheets are associated with bun-shaped pahoehoe

underlying thick massive lava. As such, compound pahoehoe

lobes several decimeters to meters across and 0.3-2 m thicklobes develop in the peripherals of and paving larger sheet(Fig. 12). Unlike pillow lobes, pahoehoe lobes have a bun-flows, just like those observed on lobate sheet flows fromlike or an amoeboidal shape and lacks corrugations on thesubmarine rift zones off Hawaii Islands and the East Pacificsurfaces which are characteristic scars formed on the surfaces

Rise (Tominaga and Umino, 2010; Umino et al., 2000; 2002;of pillow lobes as they grow (Ballard and Moore, 1977;2008).

p.17

STOP-6. Sheeted dike complex

Fig. 13. Sheeted dikes along Wadi Suhayli
The sheeted dike complex comprises almost 100%
mutually

intrusive

subparallel

dikes

with

lava

thinner than the total thickness (1.1–1.5 km) of the dike

andcomplex. Aphyric dikes predominate in the sheeted dikes, of

volcaniclastic screens increasing upward in the sheeted dikes

which 99% are simple, while multiple and composite dikes- effusive transition, and gabbroic screens increasing

are few. The thickness of dikes ranges from <1 cm to >13 m,

downward in the transition to the layered gabbros (Fig. 13)

with an average thickness of 71.3 cm. Most dikes have a bulk

(Umino et al., 2003). Transition zones are 50–100 m and 30–

Mg# of 55–66, which overlaps the majority of MORB.

50 m in thickness respectively, that are generally much