Journal: Sedimentology

Running head:

Grain-dominated, scour-fill and event-bed deposits in Arab-D carbonates

Boulder-sized carbonate event-bed deposits

Title:

Geometry, spatial arrangement, and origin of carbonate grain-dominated,

scour-fill and event-bed deposits: Late Jurassic Jubaila Formation and

Arab-D Member, Saudi Arabia

Authors:

Carl Jacquemyn1, Matthew D. Jackson1, Gary J. Hampson1, Cédric M. John1, Dave L. Cantrell2, Rainer

Zűhlke2, AbdulJaleel AbuBshait2, Robert F. Lindsay2, Ronald Monsen2

1 Dept. of Earth Science & Engineering, Imperial College London, UK2 Saudi Aramco, Dhahran, Saudi Arabia

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1. Abstract

Outcrop analogues of the Late Jurassic lower Arab-D reservoir zone in Saudi Arabia expose a

succession of fining-upward cycles deposited on a distal middle ramp to outer ramp setting. These

cycles are interrupted by erosional scours that incise up to 1.8m into underlying deposits and are

infilled with intraclasts up to boulder size (1m diameter). Scours of similar size and infill are not

commonly observed on low-angle carbonate ramps. Outcrops have been used to characterise and

quantify facies-body geometries and spatial relationships. The coarse grain-size of scour fills

indicates scouring and boulder transport by debris or hyperconcentrated density flows strengthened

by offshore-directed currents. Longitudinal and lateral flow transformation is invoked to produce the

“pit-and-wing” geometry of the scours. Scour pits and wings erode up to 1.8m and 0.7m deep

respectively and are on average 50m wide between wing tips. The flat bases of the scours and their

lack of consistent aspect ratio indicate that erosion depth was limited by the presence of cemented

firmgrounds in underlying cycles. Scours define slightly sinuous channels that are consistently

oriented N-S, sub-parallel to the inferred regional depositional strike of the ramp, suggesting that

local palaeobathymetry was more complex than commonly assumed. Weak lateral clustering of

some scours indicates that they were underfilled and reoccupied by later scour incision and infill.

Rudstone scour fills required reworking of material from inner ramp by high-energy, offshore

directed flows, associated with storm action and the hydraulic gradient produced by coastal storm

setup, to generate erosion and sustain transport of clasts that are generally associated with steeper

slopes. Quantitative analysis indicates that these coarse-grained units have limited potential for

correlation between wells as laterally continuous, highly-permeable reservoir flow units, but their

erosional and locally clustered character may increase effective vertical permeability of the Arab-D

reservoir zone as a whole.

2. Introduction

Coarse-grained scour fills, containing grains larger then several centimetres in diameter, are

uncommon in low-angle ramp settings. Outcrops of the Late Jurassic Jubaila Formation and overlying

Arab-D Member in central Saudi Arabia expose a low-angle epeiric ramp succession that contains

multiple channelised, coarse-grained scour fills containing intraclasts up to boulder size in a distal

middle ramp to outer ramp depositional setting. Channelised coarse-grained deposits usually occur

on relatively steep slopes (>2°, commonly >20°) as a result of debris flows (overview in Janson et al.,

2011). Mullins and Cook (1986) report that on low angle (<4°) slopes, aprons of debris flows that

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form sheets along depositional strike are constructed, rather than erosional channels, which

commonly form on steeper slopes. However, rare examples of channelised floatstone have been

reported in low-angle ramp settings. For example, Phelps et al. (2008) documented mega-breccia

and floatstone deposits formed by levee-slumping and debris flows along the axes of channel-levee

complexes. Additionally, in some cases, relative sea level fall has juxtaposed distal ramp deposits

with channelized shallow high energy deposits (Rankey, 2003).

The Arab-D carbonates equivalent in age to the studied outcrop succession comprise the most

prolific reservoir interval (Warren, 2006) and host several of the largest oilfields in the world (e.g.

the Ghawar, Qatif, Abqaiq and Khurais fields of Saudi Arabia; Wilson, 1981; Durham, 2005).

Understanding vertical and lateral variations in facies and associated heterogeneities in the interval

are of great importance for reservoir characterisation, modelling and management (Mitchell et al.,

1988; Handford et al., 2002; Lindsay et al., 2006). Such variations have generally been considered to

be least pronounced in the lower part of the Arab-D limestones, which comprises rhythmically

intercalated grain-dominated and mud-dominated beds (Mitchell et al., 1988) that were deposited

in a distal, storm-dominated ramp setting (Meyer and Price, 1993; Handford et al., 2002; Lindsay et

al., 2006). Although these intercalated beds occur in similar vertical successions in neighbouring

wells, their lateral continuity and geometry between wells is below the resolution of seismic and

reservoir monitoring data, and thus cannot be evaluated directly in the subsurface. Exposures near

Riyadh allow the facies architecture of approximately time-equivalent, analogous deposits

containing coarse-grained scour fills to be characterised at the inter-well scale.

The aims of this paper are threefold: (1) to present quantitative descriptions from outcrop of the

geometry and spatial arrangement of grain-dominated beds in a lower Arab-D reservoir analogue,

(2) to use these data to understand the depositional processes of coarse-grained scour fills in distal

middle ramp and outer ramp settings, and (3) to consider the potential impact of such scour fills for

characterisation of the lower Arab-D limestones and similar distal middle ramp and outer ramp

reservoirs.

3. Geological context and depositional setting

The Middle-to-Late Jurassic strata of the Arabian Peninsula are characterised by a succession of

platform carbonates deposited on the Arabian Platform, which was located just south of the

equator. These strata record deposition on a vast (2000 by 1000 km), relatively stable carbonate

shelf, which extended from present-day central Saudi Arabia to the Zagros Mountains in the east

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and central Iraq in the north, along the southern margin of the Tethys Ocean (Figure 1). The Arabian

Shield formed the western, landward boundary of the Arabian Platform, and its northeastern and

southeastern, seaward margins faced the Tethys Ocean. Several intrashelf basins (the Gotnia Basin

in the north and the Arabian and Rub’ Al Khali Basins in the centre of the Arabian Platform) were

formed in response to Early-to-Middle Jurassic rift-related differential subsidence, which influenced

gross palaeogeographic trends that persisted throughout the rest of the Jurassic (Ziegler, 2001).

These intrashelf basins bound broad shallow ramp areas on which the carbonates of the Arab

reservoirs (e.g. in the Arab-D reservoir interval; Figure 1) were deposited (Ziegler, 2001). The

intrashelf basins were interconnected, which allowed storm waves and oceanographic currents to

travel far into the Arabian Platform interior (Lindsay, 2014). The peripheral highs surrounding the

basins are interpreted to have been locations for the initiation of stromatoporoid and coral banks

and ramp-crest skeletal-oolitic shoals, which subsequently prograded into large parts of the adjacent

intrashelf basins. The Arabian Platform straddled the palaeo-equator (Figure 1) and the palaeo-

climate was hot and arid, similar to the present climate on the Arabian Peninsula (Handford et al.,

2002). The exact position relative to the palaeo-equator is not well constrained, and ranges from 10

degrees North to 15 degrees South of the equator (Murris, 1980; Sharland et al., 2001; Stampfli and

Borel, 2002; Lindsay et al., 2006; Golonka, 2007; Barrier et al., 2008).

Middle-to-Late Jurassic strata of the Arabian Platform comprise five formations (Figure 2). The

Tuwaiq Mountain and Hanifa formations were deposited during long-term Callovian-Oxfordian

transgression, when the Arabian and Rub’ Al Khali intrashelf basins formed (Sharland et al., 2001).

Maximum water depth likely reached 150 m in the intrashelf basins, where Hanifa Formation source

rocks were deposited (Droste, 1990; Sharland et al., 2001). The overlying Jubaila Formation was

deposited during the Early Kimmeridgian, and infilled most of the accommodation of the intrashelf

basins (Sharland et al., 2001). The formation records open-marine conditions that were periodically

influenced by storm activity (Goldring et al., 2005). The Arab and Hith formations record subsequent

long-term regression during the Late Kimmeridgian (Sharland et al., 2001). The alternation of

carbonates and evaporites from the lower to the upper part of each member of the Arab Formation

records high-frequency, regressive-transgressive cycles superimposed on this long-term regression

(Sharland et al., 2001; Handford et al., 2002). The Arab-D Member is marked by shallow water, high-

energy deposits that pass upwards into subtidal-to-intertidal, restricted or hypersaline deposits

(Handford et al., 2002). An idealised depositional model of the Arab-D reservoir zone is shown in

Figure 3 (after Lindsay et al., 2006).

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4. Dataset and methodology

The outcrops are located west of Riyadh (Figure 1) and expose rocks analogous to Arab-D reservoir

zone 3A and parts of zone 2A (Figure 2). The study area contains two sub-vertical, linear and

continuous road-cut exposures on either side of the Mecca-Riyadh highway (Figure 4, Figure 5A-B).

The northeastern limit of the road-cut exposures is contiguous with partly scree-covered exposures

along the southern cliff face of Wadi Laban (Figure 4, Figure 5C). There is a gentle (c. 2˚) tectonic dip

of the strata towards the south (Figure 5A-B), such that the lower interval of the succession is more

completely exposed in the northern part of the study area, and the upper interval is exposed in the

southern part of the study area.

Sedimentary logs were measured at four different locations along the outcrop, one on either side of

the Mecca-Riyadh highway and two in Wadi Laban (Figure 6). The sedimentary logs along the Mecca-

Riyadh highway road-cut exposures are not measured vertically, but instead run along the 800 m

stretch of road (Figure 5A-B). The logs span a stratigraphic height of up to 35 m, and were used for

facies analysis of the stratigraphic interval. All of the exposures contain a prominent, stratabound,

strongly karstified interval that follows a Jurassic stratigraphic layer, forms a stratigraphic datum

throughout the study area.

A three-dimensional (3D) digital outcrop model of the outcrops was created from a combination of

lidar (LIght Detection And Ranging) scans and photogrammetric reconstruction, to achieve detailed

and accurate measurements of sediment-body geometries and distributions. High-resolution lidar

scans (with an average of 10000 points/m2) were available along the road-cut exposures, and then

triangulated and coloured with high-resolution photographs (with an average of 48000 pixels/m2) to

facilitate facies identification. Photogrammetry was used to construct a 3D outcrop model of the

neighbouring wadi cliff faces, using 1600 photos collected from an average distance of 150 m from

the cliff-face exposures. Every part of the cliff faces was covered by photographs from at least five

different camera positions. The resulting photorealistic point cloud model has an average resolution

of 200 points/m2 and is georeferenced to the coordinate system of the road-cut exposures with an

error of less than 0.1 m. The final digital outcrop model covers 75000 m2 of rock outcrop in total.

Surfaces bounding different facies (e.g. erosional bases of rudstone-floatstone scour-fill units, as

described in a later section) were traced along the outcrops on the 3D digital outcrop model (e.g.

Figure 7). The spatial accuracy of the traced surfaces is estimated to be better than 3 cm. In total

more than 7.6 km of surfaces were traced out on the model with an average trace length of 334 m.

Geometrical parameters that characterise sediment-body shapes and dimensions, such as width and

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thickness, were then measured from traces of their bounding surfaces. It is important to note that

for the two road-cut exposures, these measurements capture 2D observations on sub-vertical

planes. Correlation of surfaces between the two road-cut exposures, which are separated by a

distance of c. 50 m across the highway, was carried out by visual comparison of geometrical patterns

along traces at corresponding stratigraphic levels (Figure 8). These correlations indicate

geometrically similar features on either side of the highway (e.g. erosional scours) and the overall

orientation of these features within the volume of the digital outcrop model, and thus provide some

3D geometrical information. For all observed geometrical features that could not be correlated with

confidence between the two road-cut exposures, only two-dimensional (2D) geometrical parameters

were obtained.

In addition to the geometry and dimensions of sediment bodies, their spatial distribution was also

quantitatively characterised using Besag’s L function (Besag, 1977), a variance-stabilised version of

Ripley’s K function (Ripley, 1977). Both Ripley’s K function and Besag’s L function are spatial point

process methods for detecting deviations from spatial homogeneity (Ripley, 1977), which describe

how point pattern distributions change with scale. The centroids of sediment bodies (in this case

rudstone-floatstone scour-fill units, as described in a later section) were analysed in the two road-

cut exposures (cf. channelised sandbody centroids in outcrop faces, as characterised by Hajek et al.,

2010; Flood and Hampson, 2015). The road-cut exposures are laterally extensive (800 m) but have

limited vertical extent (<30 m), such that only lateral (1D) distribution of sediment-body centroids

was calculated. Ripley’s K function, K(h), compares the number of points (sediment-body centroids)

within a lateral distance h of each point to the average rate of the point process over the studied

domain (total number of sediment-body centroids / total domain). Besag’s L function compares the

K function to its expected value and against a benchmark of zero:

L (h )=h− L2n2

∑i=1

n

∑j=1 , j ≠i

n

wi k ij for h>0

where L is the total length of the study area, n is the number of points, wi is a weighting factor to

correct for edge effects (Diggle, 1983) and kij is 1 if points i and j are a distance h or less apart, or 0

otherwise. Further distortion by edge effects is avoided by limiting the maximum value of h to 250 m

(c. 25% of the total domain length). The condition of complete spatial randomness is calculated by

200 Monte Carlo simulations, and the envelope for randomness is determined as the 95%

confidence interval of these simulations (Besag and Diggle, 1977). If the L function is lower or higher

than the envelope for randomness, then the points (sediment-body centroids) are clustered or

regularly spaced, respectively.

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5. Lithofacies analysis

Lithofacies in the Jubaila Formation and Arab-D Member have been described previously by several

authors based on subsurface data (Mitchell et al., 1988; Meyer and Price, 1993; Handford et al.,

2002; Al-Awwad and Collins, 2013) and at outcrop (Okla, 1986; Meyer et al., 1996; Meyer, 2000;

Goldring et al., 2005). The facies analysis and associated sedimentary environments presented below

is therefore brief, and follows the classification of Lindsay et al. (2006), who describe six lithofacies,

and their respective environments of depositions, in the whole Jubaila and Arab-D interval (Fig. 3). 1)

The Anhydrite lithofacies contains nodular to massive anhydrite and is interpreted to have formed in

a salina environment. This lithofacies caps each of the Arab Members. 2) The Skeletal-Oolitic

lithofacies typically consists of well-sorted grainstone and mud-lean packstone deposited in the

ramp-crest shoal environment. 3) The Stromatoporoid-Red Algae-Coral lithofacies is characterised by

poorly sorted domed, digitate and encrusting stromatoporoids, corals, micritised grains and

microbial encrustations, deposited in a proximal middle ramp environment characterised by

stromatoporoid and coral biostromes and mounds. Biostromes and mounds are reworked locally to

form floatstone and rudstone fabrics with grainstone and mud-lean packstone matrix. 4) The

Cladocoropsis lithofacies consists of limestone and dolomite containing more than 10% Cladocoropis

mirabilis that populated the intermound sheltered areas and areas upslope of the biostromes and

mounds. Cladocoropsis rudstone and floatstone contain a grainstone to mud-lean packstone matrix.

5) The Bivalve-Coated Grain-Intraclast lithofacies occurs as rudstone and floatstone with grainstone,

mud-lean packstone and most commonly mud-rich packstone matrix deposited in a distal middle

ramp setting. Dominant grain types are bivalves, coated grains, micritised grains and intraclasts,

including Cladocoropsis, stromatoporoids, corals and mudclasts. 6) The Micritic lithofacies consists of

mud-dominated limestone deposited in distal middle-ramp and outer-ramp environments, typically

capped by firmgrounds and penetrated by Thalassinoides burrows that overprint sedimentary

textures. The last two lithofacies make up the majority of the studied succession, and they are

described and interpreted in more detail below based on observations from the studied outcrops.

51. Facies characterization

The studied outcrops expose a 30 m thick succession of interbedded mud-dominated (Facies 6

above) and grain-dominated (Facies 5 above) lithofacies that occur as thin (0.5-1 m) fining-upward

cycles (Figure 9A) (Handford et al., 2002). Each cycle typically comprises a 10-20 cm thick grainstone-

to-rudstone lower part that contains peloids, bivalves, muddy intraclasts and, locally,

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stromatoporoid and coral fragments, which fines upward into wackestone and mudstone. The mud-

dominated portion of each cycle contains few grains, mostly peloids and bivalves, and is intensely

reworked by Thalassinoides burrows. Swaley cross-stratification occurs locally in both mud and ‐grain-dominated beds, and is commonly obscured by bioturbation. Cycles are separated by sharp,

commonly erosional, bases of varying relief (Figure 9B), which causes cycle thickness to vary

laterally. The top of each cycle is capped by a firmground, typically marked by prominent

Thalassinoides burrows (Figure 9B). Cycles that have a well-developed firmground top are less

affected by erosion at the base of the overlying cycle. The grain-dominated and mud-dominated

parts of the cycles are similar to respectively the ‘bivalve-coated grain-intraclast’ and ‘micritic’

lithofacies of Lindsay et al. (2006) in zones 2B, 3A, 3B and 4 in the Arab-D reservoir of the Ghawar

Field.

Chaotically bedded conglomeratic rudstone intervals up to 1-2 m thick occur locally, and contain

large (up to 1 m diameter) clasts of stomatoporoids and corals (Figure 9C-E). Other grain types

include gastropods, bivalves, peloids and rare reworked Cladocoropsis mirabilis clasts. The vast

majority of coral and stromatoporoid clasts are overturned and many are heavily bored, although a

very few examples were found in growth position. The chaotically bedded intervals infill relief that

locally incise down several metres into underlying beds or underlying scour infill. The deposits may

infill several individual incisions that are overlain by a single amalgamated (1-2 m) unit of coarse-

grained debris (Figure 9D-E). Cladocoropsis is relatively rare (Meyer et al., 2000) in these outcrops, in

contrast to Cladocoropsis-rich rudstones of lithologically similar characteristics (grain size, sorting,

fabric),in the Ghawar Field reservoir (Lindsay et al., 2006).

52. Interpretation

The fining-upward cycles represent the majority of strata. The high overall intensity of bioturbation

in the mud-dominated lithofacies suggests slow sediment accumulation in quiet, low-energy

conditions beneath fair-weather wave base, away from strong, continuous wave or current action.

The presence of swaley cross-stratification in both mud dominated and grain dominated beds ‐ ‐suggests occurrence of strong oscillatory or combined flows (Duke, 1985). The grain-supported

portion of the cycles, which overlie sharp erosional surfaces, indicate occurrence of high-energy

conditions, sufficiently strong to locally erode and rework underlying cycles.

The grain- to mud-dominated fining-upward cycles are interpreted as deposited on the distal middle

ramp below fair-weather wave base and above storm wave base where weak episodic flows

reworked small muddy intraclasts, coated grains and bivalves to form the grain-dominated portion

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(Figure 3). These episodic flows have been interpreted as storm-induced by several authors

(Handford, 1986; Mitchell et al., 1988; Meyer and Price, 1993; Goldring et al., 2005; Lindsay et al.,

2006; Al-Awwad and Pomar, 2015). No present-day analogues exist in terms of similar platform size

to the Arabian plate and facing a very large open ocean (the Thethys). Taking the modern Arabian

Gulf as an imperfect analogue indicates that storm wave base in the region can be as deep as 40 to

70 m (Sarnthein, 1970). In this and other modern analogues, bottom sediments have been found to

be stirred at water depths of up to 50 m by annual storms and up to 100 m by 100-year storms

(Lindsay et al., 2006). The same range of storm wave base depth is observed on either sides of

Florida down to 50m in protected settings (Gulf of Mexico) and down to 100m in more exposed

setting (Atlantic Ocean) (Peters and Loss, 2012). An alternative interpretation to the storm-induced

model for such episodic flows is suggested by Al-Awwad and Pomar (2015), who attribute thin (mm-

to cm-thick) intraclast-rich beds observed in core to internal wave reworking based on studies of the

nearby Khurais field (Figure 1).

The conglomeratic rudstone infills deeply scoured surfaces with very coarse (gravel to boulder size)

conglomeratic rudstone material indicating higher-energy conditions than commonly occurring

during deposition of the grain-to-mud dominated fining-upwards sediments. Chaotic organization

suggests deposition over short timespans, i.e. short-lived events. Many of these large clasts (e.g.

corals, stromatoporoids, Cladocoropsis) were originally formed above fair-weather wave base in the

proximal inner ramp (Figure 3) and were transported to more distal locations by strong episodic

flows with an offshore-directed component (cf. Figure 3) (Lindsay et al., 2006) that were strong

enough to produce deep scours and transport large clasts. The trigger for these episodic flows is

discussed in detail below. The large clast size (up to boulder size) and lack of clear normal grading in

the rudstone is indicative of deposition from either debris flows, in which clasts are mainly

supported by a cohesive matrix, or hyperconcentrated density flows, in which clasts are supported

by frictional interaction between grains (e.g. Mulder and Alexander, 2001; Talling et al., 2012). Local

incision into underlying scour fill makes the chaotically bedded intervals appear amalgamated, but

they could be the results of multiple individual events.

The observed succession can be summarized as indicating deposition under three different regimes.

Mud-dominated cycle portions were deposited well below fair-weather waves, but occasionally

disturbed by some oscillatory currents that resulted into swaley cross-stratification. When higher-

energy events occurred, sediment was eroded and locally reworked and the portion of the cycle that

was grain-dominated is deposited. With increasing energy, deeply incised scours form, that are

infilled with the coarser material sourced from more proximal positions.

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6. Vertical stacking and lateral continuity of lithofacies and lithofacies

cycles

61. Description

The vertical stacking of lithofacies and fining-upwards cycles are illustrated in four sedimentary logs

(Figure 8), with a prominent stratabound karstified mudstone interval used as a stratigraphic datum.

The thickness of fining-upward cycles exhibits a moderate fit (R2 = 0.42 -0.89) to an exponentially

decreasing recurrence frequency with increasing vertical thickness (e.g. Figure 10). In general, the

abundance and thickness of grain-dominated lithofacies units (packstone, grainstone, floatstone and

rudstone) is higher above the datum, although there is much vertical and lateral variability. In logs A

and B (through the road-cut exposures, Figure 6), the majority of floatstone and rudstone units are

over 0.5 m thick and occur above the karstified interval. Floatstone and rudstone units in logs C and

D (from Wadi Laban, Figure 6) are generally thinner (less than 0.3 m) and occur above and below the

karstified interval. Correlation of floatstone and rudstone units between the logs is commonly not

possible, even though logs are closely spaced (50-200 m).

62. Interpretation

Cycles of grain-dominated and mud-dominated lithofacies have been attributed to episodic events

such as storms or internal waves, as outlined above (Mitchell et al., 1988; Meyer and Price, 1993;

Goldring et al., 2005; Lindsay et al., 2006; Al-Awwad and Pomar, 2015), and patterns in their vertical

stacking have been attributed to cyclical relative sea-level variations (Lindsay et al., 2006). In

analogous siliciclastic successions from low-angle ramp and shelf settings, variations in sandstone

event-bed thickness and stacking are commonly interpreted to reflect variations in the frequency

and magnitude of episodic flows (e.g. storm-wave climate), changes in relative sea-level, and the

volume and calibre of sediment that was available for transport (Dott and Bourgeois, 1982; Storms

and Hampson, 2005). Distinguishing between these various controls in vertical successions without

the potential to trace out bed-scale facies architecture over large lateral distances (10s km) is

challenging and may not be possible (Storms and Hampson, 2005). The thickness distributions of

lithofacies and lithofacies cycles in vertical successions have been used to evaluate hierarchical

stratigraphic ordering, particularly in peritidal carbonate strata (e.g. Drummond and Wilkinson,

1996; Burgess, 2008). Exponential thickness distributions of the type observed in the studied

succession are commonly interpreted to indicate deposition by a stochastic Poisson process, in

which the thickness between each pair of consecutive events (in this case, formation of cycle

boundaries and deposition of grain-dominated lithofacies) has an exponential distribution and thus

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events occur independently from each other. This thickness distribution is consistent with an

interpretation of cycle deposition driven by independent events, rather than by hierarchical relative

sea-level cycles (e.g. Drummond and Wilkinson, 1996), although it should be noted that the number

of cycles in the studied vertical succession (<35 m) is too small to provide a statistically robust result.

High-frequency relative sea-level fluctuations cannot be discounted as a control on cycle stacking,

but this mechanism implies that the cycles can be correlated over the whole inner-to-outer ramp

transect (Figure 3)(cf. Storms and Hampson, 2005) which is inconsistent with our data.

7. Rudstone-floatstone scour-fill geometry and orientation

71. Description

Figure 7 shows the distribution of rudstone along the two road-cut exposures. As noted previously,

rudstone-floatstone units overlie scoured, erosional bases with up to 1.8 m of erosional relief

(Figure 9D-E). Comparison of the geometry of the erosional bases of rudstone-floatstone scour fills

exposed in the road cut reveals recurring geometrical patterns in cross-section (Figure 11, 12, 13,

14). No complete cross-sections through rudstone-floatstone scour fills are evident in the wadi cliff-

face exposures, which are of lower quality. The erosional surfaces typically consist of a shallow

portion (scour wings), with locally incised deeper scour pits. The scour wings have near-horizontal

bases, and exhibit gradual lateral thinning to their pinchouts. Steep-walled scour pits erode on

average two times deeper into underlying beds than the scour wings. The scours incise into

underlying fining-upwards cycles and into underlying scour-fills. The latter produce amalgamated

chaotic beds.

Comparison of traced surfaces on both sides of the highway implies that some scours can be

matched, and thus projected, between the two road-cut exposures based on their similarity in cross-

section (Figure 6). Although such correlations assume that the scours are channelised and retain a

relatively uniform cross-sectional geometry over the width of the highway (c. 50 m), the method

provides a means to estimate the overall scour orientation. The orientation of the scours is tightly

clustered around a north-south trend (Figure 15), at an orientation of c. 36° to the highway, with

only a few scours oriented more than 10° away from this trend. All cross-sectional scour geometries

measured in the road-cut exposures are thus apparent widths, which are corrected to true widths

throughout this paper (Figures 11, 12, 13; Tables 1, 2) (width = apparent width * sin(36°)).

The cross-sectional geometry of the scours was quantified by 15 parameters measured directly from

surfaces traced on the digital outcrop model (Figure 12A). The average lateral extent of the scour

surfaces in cross section is 40 m, and they have an average depth of 0.5 m. The scour pits cut down

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into the underlying strata by up to 1.8 m deeper. Data characterising scour depth and width

distributions are summarised in Figure 12, and are approximated by a lognormal distribution (Table

1, Table 2). Cross-correlation between measured values for geometrical depth parameters indicates

moderate to good correlation between scour pit, pit flanks and wings (correlation coefficient, R2 =

0.71-0.95; Figure 14), but there is at best a moderate, and typically no, cross-correlation between

depth and width parameters (correlation coefficients R2 < 0.57, and on average 0.13) or for width

parameters only (correlation coefficients R2 < 0.57, and on average 0.21). The distributions and

cross-correlation of geometrical parameters indicate that the scour widths are close to symmetrical

around their vertical centrelines. Only width parameters b1 and b2, which characterise the dip of the

scour-flanks, are distinctly different. Values of b1 are approximately half those of b2, indicating slight

asymmetry in the scour pit geometry, with consistently steeper southwestern flanks than

northeastern flanks. The dips of the scour pit flanks, scour wings and scour wing tips were calculated

from the various geometrical parameters (Figure 13). The scour wings are close to horizontal

(average dip: 0.6°) and pinch out at their tips mostly at low angles (average dip: 11°), although a few

more abrupt pinchouts with angles up to 50° are also observed at scour wing tips. The scour pit

flanks are generally steeper (up to 64°); their southwestern and northeastern flanks dip on average

at 24° and 14°, respectively.

72. Interpretation

The strong correlation between depth measurements at different parts of the scour cross-sections

shows that a similar variation in cross-sectional erosional profile was established for different scour

events. This correlation implies that parameters controlling the incision depth, such as the scour

processes (e.g. debris flows and hyperconcentrated density flows) and boundary conditions (e.g.

vertical variations in substrate cohesion), did not change dramatically between flows. The scour pits

incised into fining-upward cycles capped by cemented firmgrounds that were relatively resistant to

erosion. These firmgrounds probably limited the depth of erosion of scour pits and scour wings,

which also explains why both scour pits and scour wings have nearly horizontal bases (Figure 12).

Similar inferences are drawn for other deposits generated by high-energy events in carbonate

ramps; for example, Dibenedetto and Grotzinger (2005) described storm-transported conglomerates

and breccias that lack basal erosional scours as a result of early seafloor cementation.

No correlation is observed between the width measurements of the scour cross-sections, implying

that different parameters controlled scour width (e.g. subtle local palaeobathymetry, sinuosity or

lateral variations in substrate cohesion) and that these parameters were variable between events.

This inference is consistent with the wide range of width-depth ratios for storm-generated,

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channelised gutter casts documented in other distal carbonate ramps (Jennette and Pryor, 1993). In

the same way that scour pit deepening can be halted by resistant underlying firmgrounds, lateral

scour pit erosion may reflect lateral variations in firmground continuity and degree of firmground

cementation. The lack of any link between depth and width measurements shows that the width of

scours or confinement of the flows did not affect the incision depth. It cannot be directly observed in

these outcrops whether the channelised scours have a linear or sinuous geometry in plan-view

between the two road-cut exposures, although channel curvature appears likely given that the scour

pits are consistently slightly asymmetrical (Amos et al., 2003). The more gently dipping eastern

flanks of the scour pits would have occurred on the inner bends of curved channels in this

interpretation.

8. Lateral distribution of scour pits

81. Description

The application of Besag’s L function to scour-pit centroids (Figure 7) describes their lateral

distribution along the road-cut exposures, and the results are shown in Figure 16. On the

southeastern side of the highway, scour pits are randomly distributed over the range of length scales

analysed. Scour pits on the northwestern side of the highway show weak clustering at length scales

from 50 m to 150 m. At smaller and larger length scales, scour pits are distributed randomly.

82. Interpretation

The absence of strong patterns in the spatial distribution of scour pits implies that there was no

dominant control on their position, although it should be noted that the number of datapoints for

analysis is small (n = 96). The difference in scour-pit distribution between the two road-cut

exposures (Figure 7,Error: Reference source not found) supports the interpretation than the scour

pits have a range of plan-view orientations (Figure 15) or implies that the geometry of the scour pits

changes along their axes over the distance between the two road-cut exposures (c. 50 m). Weak

lateral clustering of scour pits on the northwestern side of the highway (Figure 16B) likely reflects re-

occupation of some scour pits, either because they were underfilled and thus formed slight

palaeobathymetric lows that were used by a later flow(s), or because they were infilled by relatively

non-cohesive sediment that was easily eroded by a later flow(s). This results into more extensive and

amalgamated scour fills. Compensational stacking of scour pits, which would have resulted in their

regular spacing (e.g. Straub et al., 2009), appears to be absent. This absence indicates that the scour-

pit fill deposits did not form palaeobathymetric highs on the seabed, for example due to early

differential compaction, that deflected later flows into neighbouring palaeobathymetric lows.

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Alternatively, there may have been a long-lived, localised palaeobathymetric low near the

northwestern side of the highway that acted to route successive flows through a subtly defined

fairway. Such a fairway did not extend across to the southeastern side of the highway. This inference

is also in agreement with the common occurrence of rudstone-filled scours beneath the karstified

horizon in logs C and D, but not in logs A and B (Figure 6).

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9. Discussion

91. Formative mechanism(s) of fining-upward cycles and scour fillsThe grain-dominated parts of cycles have been interpreted previously as the result of storm-induced

flows by several authors (Mitchell et al., 1988; Meyer and Price, 1993; Lindsay et al., 2006) or as the

result of internal waves at a pycnocline (Al-Awwad and Pomar, 2015). The near-equatorial

palaeogeographic location of the Arabian Platform in the Jurassic would appear to preclude major

storm formation and thus to favour the interpretation of episodic flows generated by breaking

internal waves. However the internal-wave deposits interpreted by Al-Awwad and Pomar (2015) in

the Arab-D limestones are thinner and finer grained than the grain-dominated lower parts of fining-

upward cycles observed in the studied outcrops. Tropical depression storms do not commonly form

within 5 degrees (+/- 550 km) N or S from the equator (Anthes, 1982; Henderson-Sellers et al., 1998),

but at least two equatorial storms have recently (1956 and 2001) formed within equatorial latitudes

(Fortner, 1958; Chang et al., 2003) and many more formed at higher latitudes and then moved into

latitudes lower than 5 degrees (Lander, 1996; Yi and Zhang, 2010). Extra-tropical storms can

generate large waves that propagate into equatorial locations (Wasserman and Rankey, 2014). By

applying the principle of uniformitarianism, we can extrapolate that although less frequent than

outside of the equatorial belt, tropical storms may have caused episodic sediment transport in the

equatorial Late Jurassic ramps of the Arabian Platform. Furthermore, numerical modelling of storm

activity for the Late Jurassic (Agustsdottir et al., 1999) show the potential for storms to form in the

southern Tethys Ocean. In the stratigraphic record, preservation of Thalassinoides ichnofacies and

absence of hummocky cross-stratification and scouring have been used to constrain the position of

the palaeo-equator and a surrounding storm-free zone (e.g. Jin et al., 2012). In the outcrop

investigated, the occurrence of swaley cross-stratification and incomplete preservation of

Thalassinoides at cycle-top firmground surfaces in the studied deposits indicates that tropical storms

cannot be discounted as a mechanism to deposit the lower, grain-dominated parts of the fining-

upward cycles. The Jurassic example of Saudi Arabia is also not unique, as previous regional work on

Lower Cretaceous outcrops have suggested that storm deposition was a key factor in controlling the

extensive tidal flat deposits of East Central Oman (Immenhauser et al., 2004; Sena and John, 2013).

The conglomeratic rudstone scour fills can most probably be attributed to storms (Table 3). The large

fragments of corals and stromatoporoids in the rudstones required powerful episodic flows to

transport material downslope from well-anchored biostromes. The majority of the clasts, including

corals and stromatoporoids, originated from above fair-weather wave base, where sea water was

constantly mixed, which would have prevented the formation of a pycnocline. Conglomeratic

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rudstones of similar calibre are generally attributed to slope failures or submarine gravity flows that

form on steep slopes (>2°) (Mullins and Cook, 1986; Janson et al., 2011; Le Goff et al., 2015), and are

not associated with low-angle ramps. Storm-wave action is a common triggering mechanism of both

cohesive and frictional flows by liquefaction or suspension (Walker, 1984; Prior et al., 1989; Madsen

et al., 1993; Seguret et al., 2001; Alfaro et al., 2002; Paull et al., 2002). Storm activity is accompanied

by coastal set up or storm surge, that is particularly large on wide and low-angle shelves (Myrow and

Southard, 1996) such as the Arabian Platform shelf. Coastal setup during storms generates an

offshore-oriented hydraulic pressure gradient that results in a downslope-directed relaxation current

or rip current, which may travel the entire shelf and affect outer shelf regions below storm wave

base (Beardsley and Butman, 1974; Myrow and Southard, 1996), and that may be strong enough to

rework and transport large clasts.

Alternative mechanisms to account for the occurrence of conglomeratic rudstone scour fills appear

to be unlikely (Table 3). Although breaking internal waves can account for the generation of

abundant, erosionally based beds (e.g. grain-dominated parts of fining-upward cycles, as outlined

above), examples interpreted in Arab D carbonates lack deep erosional relief (Al-Awwad and Pomar,

2015). In addition, such beds cannot contain clasts reworked from water depths that are shallower

than the pycnocline; this is hard to reconcile with the occurrence of coral and stromatoporoid clasts,

formed in shallow, wave-agitated waters, in the rudstone scour fills. Tsunamis can displace boulder-

sized clasts, but the abundance of superimposed rudstone-filled scours would require that the same

area of the Arabia Platform was hit repeatedly by frequent tsunamis. Tectonic activity was

uncommon during the Late Jurassic development of the Arabian Platform (Sharland et al., 2001;

Ziegler, 2001; Handford et al., 2002), and cannot be invoked as a trigger for tsunamis, or as a way of

generating steep ramp angles or breaks in slope. Relative sea-level falls could have been responsible

for eroding and reworking large clasts, but they do not provide a mechanism for long distance

transport on a low-angle ramp. Furthermore, in this interpretation, the occurrence of floatstone-

rudstone scour fills at multiple stratigraphic levels would require abundant, high-frequency relative-

sea-level falls that are not supported by sub-regional sequence stratigraphic correlation (Al-Awwad

and Collins, 2013).

92. Erosion and sediment transportThe coarse-grained composition of the conglomeratic rudstone scour fills are typical for debris flows

or hyperconcentrated density flows (e.g. Mulder and Alexander, 2001; Talling et al., 2012). The

former have long run-out distances, which may reach hundreds of kilometres, due to aquaplaning

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(e.g. Mohrig et al., 1998; Gee et al., 1999), whereas the latter are gravity driven and cannot normally

be maintained at very low angles or for long run-out distances (e.g. Shanmugam, 1996; Talling et al.,

2012). Lindsay et al. (2006) interpreted rudstone beds with mud-rich matrix as the products of debris

flows, and mud-lean rudstone beds as hyperconcentrated density flows. Both types of flow tend to

be non-erosional or only locally erosional (Talling et al., 2007), although erosion can be important in

maintaining flow density and thus sustaining flows with long run outs (Mulder et al., 1997; Morris et

al., 1998; Piper and Normark, 2009). Most large (bio)clasts in the rudstone beds had a primary

intraparticle and framework porosity, such that their average density would have been relatively low

and they could have been transported by concentrated, rather than hyperconcentrated, density

flows.

The combination of either a debris flow or a hyperconcentrated density flow and the extra

momentum of a relaxation rip current would have resulted in a more powerful downslope-directed

flow with the potential to travel long distances even on a very low-angle slope (Myrow and

Southard, 1996). The high flow velocities of such combined flows would have promoted dilution and

thus transformation into concentrated flows with greater erosional potential than

hyperconcentrated flows (Mulder and Alexander, 2001). The resulting flow may have consisted of a

concentrated flow head, which formed an erosional scour, followed by a debris or

hyperconcentrated flow body, which infilled the scour.

The presence of rip-up clasts in the grain-dominated portions of fining upward cycles indicates scour

formation also resulted in reworking and transport of material from underlying fining-upwards

cycles, which was transported and redeposited further downslope, where erosion was limited as the

flow decelerated. The grain-dominated portions of fining-upward cycles are thus likely to be the

distal expression of coeval rudstone-filled scours in more proximal locations.

93. Scour geometryThe observed rudstone-filled scour geometries provide insights into flow conditions and controls on

the erosional flows that transported sediment downslope across the low-angle ramp. The width and

relatively gentle lateral thinning of the scour wings is similar to the characteristics of most modern

and ancient storm-related scours described in the literature. Subtidal scour channels associated with

rip currents in a Triassic carbonate ramp are 1-30 m wide and up to 1 m deep (Aigner, 1985). A wide

range of width-depth ratios is reported for deeply scoured, channelised gutter casts generated by

locally focussed storm-generated flows in a distal Ordovician carbonate ramp (Jennette and Pryor,

1993). Scours up to 120 m wide and 3 m deep, with steep to overhanging walls, are attributed to

debris flows in gravelly, siliciclastic fan-delta deposits of Miocene age (Sohn, 2000). Storm-induced

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shore-perpendicular channels and gutters in a modern, sandy siliciclastic shoreface are 20-60 m wide

and up to 0.5 m deep, with a shore-parallel spacing of 50 m (Amos et al., 2003). These channels

widen offshore to up to 300 m, due to flow transformation. However, the observed geometry of

deeply eroded scour pits that extend laterally into scour wings is different from most submarine

channels, which are surrounded by levees that record aggradational deposition that is elevated

above the channel margins (Keevil et al., 2007).

The observed “pit-and-wing” geometry of the scours can be explained by lateral flow

transformations. The centre of the flow had less interaction with the surrounding seawater, and thus

reached the highest velocities and generated the deepest erosion. This interpretation is consistent

with the concentration of most boulder-sized clasts inside scour pits. With increasing distance from

the scour centre, more water was entrained in the flow, and flow density and erosional potential

decreased. Erosion rates are expected to be different for different flow densities. En masse settling

or freezing of density flows is not instantaneous and material at the sides of flows comes to a halt

first while the centre of the flow can continue to move (Middleton and Hampton, 1973; Iverson,

1997) and produce deeper scour pits.

The pronounced north-south trend of the scours (Figure 15) is not consistent with an onshore-to-

offshore direction in the interpreted regional palaeogeographic context (Figure 1) of a (now eroded)

palaeo-shoreline to the west and intrashelf basins to the east (cf. Leckie and Krystinik, 1989; Myrow

and Southard, 1996). Local palaeogeographic context is based on subaqueous dune orientation and

flat pebble imbrication in Wadi Nisah, suggesting west-east orientation of the scours 50 km south of

these outcrops (Meyer et al., 1996). On modern shelves, offshore-directed storm currents are

typically deflected by pressure and excess-weight currents due to Coriolis forces, to form shore-

parallel geostrophic currents (Swift et al., 1986; Snedden et al., 1988). In the Late Jurassic palaeo-

equatorial setting of the Arabian Platform, such Coriolis-driven deflection would be minimal, and

thus the orientation of storm relaxation currents would only have been controlled by sea-bed

palaeobathymetry and palaeo-coastline orientation. It is thus likely that a more complex palaeo-

coastline morphology and/or local palaeo-bathymetry existed than is portrayed in regional

palaeogeographic maps (e.g. Figure 1), and that flows were consistently deflected from the expected

west-to-east orientation of the regional palaeoslope to a north-south orientation. Scour pits

consistently have a slightly steeper western side than eastern side (Figure 12C), which could reflect

consistent curvature of the scour path such that the eastern scour-pit flank always lay on the inside

bend of a curved channel (e.g. Sohn, 2000). This subtle but consistent direction of cross-sectional

asymmetry and associated, inferred plan-view curvature does not support the notion that the scour

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thalwegs were slightly sinuous (cf. Amos et al., 2003), but instead favours a consistent flow

deflection, for example by a local palaeobathymetric high that persisted throughout deposition of

the succession.

94. Implications for reservoir characterisationThe continuity, geometry and spatial distribution of lithofacies in the outcrops indicates that there is

likely to be significant vertical and lateral heterogeneity at inter-well scales in Arab-D limestone

reservoirs, and more generally in storm-dominated, middle-to-outer carbonate-ramp reservoirs.

Much of the heterogeneity at outcrop is defined by variations in the thickness and extent of grain-

dominated event beds and intervening mud-dominated deposits, which are paired to form

lithofacies cycles. Individual beds and lithofacies cycles vary in their lateral extent (Figure 7), such

that they are difficult to correlate even between closely spaced (c. 50-200 m) vertical sections that

contain superficially similar vertical patterns in bed and cycle stacking (Figure 6). Such lateral

variation, in combination with the exponential distribution of cycle thickness (Figure 10) is consistent

with weakly channelised deposition by storm events that exhibited stochastic variability in their

magnitude and frequency.

Heterogeneity is most pronounced for floatstone-rudstone scour fills, which are likely to have the

highest depositional porosities and permeabilities in reservoir zones analogous to the studied

outcrops (Clerke et al., 2008). The average total width of the scour fills in cross section is

approximately 50 m, and they extend along their axes for at least a similar distance in a consistent

orientation (north-south; Figure 15) that is hard to reconcile with, and predict from, regional

palaeogeographic reconstructions (e.g. Figure 1). Floatstone-rudstone beds penetrated by wells

should not be used as marker beds for correlation, but are instead likely to be associated with

significant variation, and perhaps also anisotropy, in horizontal permeability as a result of local scour

orientation and lateral scour pinch outs.

The floatstone-rudstone scour fills have a “wing-and-pit geometry” (Figure 12), and are

characterised by deep (up to 1.8 m) erosion into underlying beds. Locally, they also display some

weak lateral clustering (Figure 16). The floatstone-rudstone beds penetrated in wells may be locally

well connected vertically with each other and with other grain-dominated beds, such that bed-scale

permeability variation is not simply layered. Instead, effective vertical permeability in the reservoir

zones that contain floatstone-rudstone beds is likely to be variable and may be high in certain

locations. In summary, the observed geometry and spacing of floatstone-rudstone beds suggests

that they may be locally connected to form conduits for lateral and vertical fluid flow, especially

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where they intersect fractures or their permeability is enhanced by dolomitisation (cf. Meyer et al.,

2000; Cantrell et al., 2001).

10. Conclusions

Outcrops of the Late Jurassic Jubaila Formation and Arab-D Member in central Saudi Arabia expose a

distal-middle-to-outer ramp succession, deposited below fair-weather wave base, which lay close to

the palaeo-equator. The architectural characteristics of this succession give insights into the

geometries, spatial arrangement and formative mechanism(s) of metre-scale fining-upward cycles

and coarse-grained scour fills in carbonate ramps, as well as providing data to constrain reservoir

character in inter-well volumes of the prolific Arab-D reservoir interval.

The succession consists of metre-scale fining-upward cycles which are stacked vertically, punctuated

by rudstone scour fills. Erosionally-based cycles consist of a grain-supported lower portion that fines

upward into wackestones and mudstones, and are capped by cemented firmgrounds. Each cycle

represents an independent event such as a storm or episodic flow generated by breaking internal

waves. Variations in cycle thickness and stacking are consistent with stochastic variations in the

magnitude and frequency of such events, although the additional influence of relative sea-level

changes cannot be excluded. Chaotically-bedded conglomeratic rudstones contain coarse clasts up

to 1 m in diameter, and infill scours that incise up to 1.8 m into underlying fining-upwards cycles.

These rudstone scour fills are the result of strong storms that reworked stromatoporoids, corals and

Cladocoropsis on the inner ramp and initiated transport to a more distal position. Transport of these

large clasts occurred by storm-initiated debris flows or hyperconcentrated density flows that had

increased strength due to coastal storm setup and resulting downslope rip currents. This mechanism

enables coarse-grained conglomeratic rudstones to be deposited on the distal-middle-to-outer parts

of low-angle ramps.

Incision of the rudstone-filled scours is the result of transformation of debris flows or

hyperconcentrated density flows at the flow head into strongly erosive concentrated density flows.

The scours have a “pit-and-wing” geometry in cross section that is probably the result of lateral flow

transformation, and form curved channels in plan view. The incision depth of a scour pit and its

wings (respectively up to 1.8 and 0.7 m) and the scour-pit and wing-tip-to-wing-tip widths

(respectively up to 10 and 55 m) are affected by the cementation of the underlying substrate. The

vertical distribution of firmgrounds capping the underlying fining-upwards cycles limits scour depth.

Scour width is influenced by the lateral continuity of the firmgrounds. There is a wide range of

resulting aspect (width:thickness) ratios. Scour fills do not show compensational stacking, but some

localised lateral clustering is attributed to underfilling and re-occupation of the scours. The

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channelised scours have a north-south orientation, which implies deflection by subtle, localised

palaeobathymetry of the ramp, in the absence of Coriolis forces near the equator.

11. Acknowledgements

The authors thank Saudi Aramco supporting and allowing us to publish this work. In particular, we

would like to acknowledge Maher Al-Marhoon, Chief Technologist of the Geology Technology

Division and Waleed Al-Mulhim, EXPEC-ARC Manager for their support. We thank Gene Rankey,

Xavier Janson, Gregor Eberli, Tracy Frank and Peir Pufahl for their insightful reviews and editorial

comments.

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13. Figures

Figure 1. Palaeogeographic map of the Arabian Platform during the late Jurassic (modified from Handford et al., 2002; Lindsay, 2014). The location of the studied outcrops is indicated by a red star, west of Riyadh.

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Figure 2. Generalised Upper Jurassic stratigraphy for the Arabian Platform. The interval exposed in the selected outcrops (equivalent to Arab-D reservoir zones 2B and 3A in the subsurface) is indicated in the red box (modified from Powers, 1968; Meyer et al., 1996).

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Figure 3. Idealised depositional model of the Arab-D reservoir carbonate ramp in the Ghawar Field, modified from Lindsay et al. (2006) with associated lithofacies classification. The studied outcrops mainly expose deposits of the distal part of the middle ramp and the outer ramp (red box).

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Figure 4. Aerial photograph of the outcrops, which consist of continuous road-cut exposures (red and blue) on either side of the Mecca-Riyadh highway and neighbouring cliff-face exposures (yellow and green) in Wadi Laban. The latter exposures are partly scree covered. The study area is located west of Riyadh in central Saudi Arabia (Figure 1). Location of two vertical sedimentary logs in Wadi Laban are indicated by blue circles.

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Figure 5. Photographs illustrating exposure quality and continuity along (A, B) the Mecca-Riyadh highway road cuts looking south (red in Figure 4) and north (blue in Figure 4) respectively and (C,D) the Wadi Laban cliff faces, northwest (green in Figure 4) and southeast (yellow in Figure 4) of the highway respectively. A stratabound karstified interval forms a prominent marker unit used for correlation (indicated by ‘K’).

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Figure 6. Correlation of four sedimentary logs, highlighting coarse-grained (floatstone and rudstone) lithofacies units in orange. Logs A and B were measured along road-cut exposures, and logs C and D were measured from cliff-face exposures in Wadi Laban. Blue lines are used to indicate potential correlation of coarse-grained lithofacies units between logs. A prominent, stratabound karstified unit (Figure 5) (labelled by encircled K) is used as a datum. (M: mudstone; W: wackestone; P: packstone; G: grainstone; F: floatstone; R: rudstone)

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Figure 7. Rudstone distribution along road-cut exposures along the (A) southeastern and (B) northwestern sides of the Mecca-Riyadh highway (red and blue in Figure 4, respectively).The location of sedimentary logs A and B (Figure 6) are

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shown. The centroid of each rudstone-floatstone scour fill is shown as a red point, to be used for spatial analysis using Besag’s L function.

Figure 8. Example of correlation between two traces of a stratigraphic surface in road-cut exposures on both sides of the Mecca-Riyadh highway (Figure 4). The central part of the figure shows two surface traces A and B over their full extent in the digital outcrop model, coloured to show variation in the their vertical position relative to the overall trend of the surface (thin grey line). Road-cut exposures on the northwestern side (trace A) and southeastern side (trace B) of the highway are separated by c. 50 m. Black-bordered parallelograms between the two traces highlight distinctive patterns that are used to correlate geometrical features across the highway. These distinctive patterns are enlarged for each parallelogram, and shown in black-bordered rectangles in the upper and lower parts of the figure.

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Figure 9. Photographs of facies characteristics in the studied outcrops (Figs. 4, 5). A) Vertically stacked fining-upward cycles. Arrows indicate cycle boundaries. Each cycle comprises a grain-dominated lower part and a mud-dominated upper part, and is variably reworked by bioturbation. B) Erosional contact between cycles. The upper part of the underlying cycle comprises moderately bioturbated mudstone-wackestone that is overprinted by grainstone-filled Thalassinoides burrows associated with a firmground at the cycle top, which was subsequently modified by erosion. C) Rudstone-filled erosional scour. Individual clasts up to 30 cm in diameter are visible inside the rudstone. D) Uninterpreted photograph and E)

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interpreted line drawing of multiple cross-cutting and amalgamated erosional scours filled with rudstone. The rudstone is overlain by packstone and grainstone.

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Figure 10. Frequency distribution of the thickness of lithofacies cycles in logs A-D (Figure 6). Cycles comprise grain-dominated lithofacies that grade upwards into mud-dominated lithofacies (Figure 9A). The cycles exhibit a moderate fit (correlation coefficient, R2 = 0.42-0.89) to an exponentially decreasing recurrence frequency with increasing vertical thickness, which is consistent with a stochastic Poisson process such as deposition by episodic storm events.

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Figure 11. A) Examples of traces along the erosional bases of rudstone-floatstone scour fills on the digital outcrop model (traces are indicated in Figure 7), and B) conceptualised cross-sectional geometries based on these observations.

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Figure 12. Overview of measurements that are used to characterise the cross-sectional geometry of the erosional bases of rudstone-floatstone scour fills. A) Conceptual cross section of a scour surface, with 15 parameters defining its shape. B) Box plots of all scour depth measurements. C-D) Box plots of all scour width measurements. The total number of measurements is 43-77 for different parameters (Tables 1, 2).

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Figure 13. Schematic cross-section through the erosional base of a rudstone-floatstone scour fill, showing rose diagrams of dip measurements at their respective positions (n = 43-65).

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Figure 14. Scatter plots of measurements for depth parameters (Figure 12B), which indicate moderate to strong correlations between successive depth parameters. A-B) Depth of scour pit centre (d0) correlates well to the depth of both scour pit sides (d1, d2). C-D) Depth correlation between the scour pit sides (d1, d2) and proximal scour wings (d3, d4) are similar for both sides of scour. E-F) Depth correlation between proximal (d3, d4) and distal (d5, d6) scour wings is strong, but the correlations indicate that slightly different angles of basal scour exist between the two scour wings.

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Figure 15. Rose diagram of the orientations of rudstone-floatstone scour fills, based on correlation of scour fills of similar cross-sectional geometry between the two road-cut exposures (Figure 8). The total number of measurements is 16.

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Figure 16. Graphs of Besag’s L function for scour-pit centroids in road-cut exposures on the (A) southeastern and (B) northwestern sides of the Mecca-Riyadh highway (red and blue in Figure 4, respectively). The 95% confidence interval around the condition of complete spatial randomness is shown in grey. Randomly distributed centroids plot in this grey interval, whereas clustered and regularly spaced centroids plot beneath and above it, respectively. A) Scour pits are distributed randomly over all observed length scales in the southeastern road-cut exposure. B) The northwestern road-cut exposure shows clustering of scour pits over lateral length scales between 50 and 150 m, but they are randomly distributed at smaller and larger length scales in this exposure. The value of L(h) is defined by equation 1.

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922

14. Tables

d0 d1 d2 d3 d4 d5 d6

n 77 74 71 71 65 52 49

µ -0.46 -0.59 -0.57 -1.44 -1.44 -1.49 -1.62

σ 0.57 0.55 0.58 0.95 0.94 0.82 0.86

Table 1. Lognormal distribution parameters for all depth measurements displayed in Figure 12A. The number of measurements (n), and the location parameter (μ) and scale parameter (σ) of the lognormal distribution are given.

a1 a2 b1 b2 c1 c2 e1 e2

n 77 77 75 69 49 43 51 45

µ 0.56 0.57 -0.15 0.44 1.98 2.11 0.47 0.48

σ 1.20 1.31 0.87 0.80 1.44 1.29 1.14 0.97

Table 2. Lognormal distribution parameters for all width measurements as displayed in Figure 12C-D. The number of measurements (n), and the location parameter (μ) and scale parameter (σ) of the lognormal distribution are given.

45

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927

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930

storm waves internal waves

tsunamis tectonic activity

relative sea-level falls

generation of metre-deep erosional scours on middle-to-outer ramp

yes no yes yes (near breaks in slope)

yes (for large sea-level falls)

erosion and offshore-directed transport of gravel- to boulder-sized intraclasts from inner ramp

yes no yes yes (near fault scarps)

yes (by lowering of fair weather wave base)

capable of generating abundant (high-frequency) events

yes yes no yes no

Table 3 Compatibility of potential mechanisms for generating conglomeratic rudstone scour fills with interpreted middle-to-outer ramp setting of studied succession (Figure 3). The characteristics of beds that were potentially generated by internal waves in the Arab-D carbonates are taken from Al-Awwad and Pomar (2015).

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931

932933934

935

936

937