Submarine slope processes in rift-margin basins ... - GeoScienceWorld

Submarine slope processes in rift-margin basins, Miocene Suez Rift, Egypt Lorna J. Strachan1,2,†, Frank Rarity1, Robert L. Gawthorpe1,3, Paul Wilson1,4, Ian Sharp5, and Dave Hodgetts1 1

School of Earth, Environment & Atmospheric Sciences, University of Manchester, Manchester, M13 9PL, UK Geology, School of Environment, University of Auckland, Auckland 1142, New Zealand 3 Department of Earth Science, University of Bergen, N-5007 Bergen, Norway 4 Rock Deformation Research Ltd., Earth Sciences, University of Leeds, Leeds LS2 9JT, UK 5 Statoil, Sandsliveien 90, Bergen, Norway 2

ABSTRACT New, high-resolution lithofacies data from hanging-wall Miocene synrift (Rudeis Formation) exposures of the eastern Suez Rift margin, Egypt, reveal a submarine slope depositional system dominated by coarsegrained (pebble), heterogeneous, lenticular beds, formed by coalescing turbidity currents, slumps, and debris flows deposited on deforming submarine substrate. Flows include prerift clasts and contemporaneous shallow-marine fossil fragments originating from an uplifted eastern hinterland. Multiple terrestrial drainages debouched onto faulted offshore slopes or fed small fan deltas on narrow shelves (360 m Rudeis Formation is divided into stratigraphic units R1 and R2, which exhibit upward-coarsening and unordered vertical motifs. Ongoing faulting influenced synrift deposition by controlling the locus of subsidence and gravity base level. Mesoscale faults became inactive during Rudeis Formation times, with strain localized on the large rift border fault system, leading to a wider basin with time. We compare 16 subaqueous rift-margin basin fills from various tectonic and geographic settings and show they generally represent proximal gravityflow deposits dominated by nongraded beds. We find little commonality in vertical grain-size trends, highlighting the diversity of stratal architectures. Most basin fills show an inverse relationship between maximum clast size and shelf width. We propose a new model to capture the spectrum of sedimentary responses to rifting within rift-margin basins, varying as a function of shelf width, slope gradient, maximum grain size, and †

E-mail: [email protected]

textural maturity. Rudeis Formation strata at north Wadi Baba represent a particularly coarse-grained end member, deposited on steep slopes, with a narrow shelf. INTRODUCTION Rifting is a fundamental plate-tectonic process that shapes Earth’s surface and results in the rupture of continental crust and formation of sedimentary basins. Extension-induced landscape evolution is controlled by faulting through footwall uplift and hanging-wall subsidence. Faulting therefore exerts a primary control on sedimentary processes and deposits by driving sedimentation, controlling fluidflow and gravity-flow pathways, and creating sites of deposition (Leeder and Gawthorpe, 1987; Gawthorpe and Leeder, 2000; Mack et al., 2009). Rift-bounding border faults are characterized by large throws, uplifted rift flanks, and corresponding subsidence of rift valleys (Ebinger, 1989). Thus, rift-margin basins, located immediately adjacent to border faults have the potential to form deep depocenters characterized by complex high topographic relief, multiple sediment sources, and a plethora of active sedimentary processes. Rift-margin basin synrift strata therefore may provide important sedimentary archives that are important hydrocarbon targets and that may provide insights into the sedimentological and structural evolution of divergent margins. Several insightful studies of rift-margin basins from the United States, Greece, and Kenya filled with terrestrial and marginal marine strata have revealed reorganization of paleorivers around growing faults and folds, with related surface tilting driving channel avulsion, re-incision, and abandonment (e.g., Mack and Leeder, 1999; Saneyoshi et al., 2006; Backert et al., 2010). Resultant synrift rock types are complex, with coarse-grained assemblages forming coalescing deposits of alluvial fans, fluvial channels, paleo-

sols, lacustrine beds, and fan deltas (e.g., Leeder et al., 1996). The reconstruction of submarine slope and basin floor sedimentary processes in riftmargin basins has received less attention, with only a relatively small number of detailed studies (e.g., Surlyk, 1978, 1984; Ferentinos et al., 1988; Papatheodorou and Ferentinos, 1993; Leppard and Gawthorpe, 2006). This has resulted in a disconnect between the largely hypothetical regional synrift tectonostratigraphic models (Prosser, 1993; Bosence, 1998; Ravnås and Steel, 1998; Gawthorpe and Leeder, 2000) that predict upward-fining resulting from increased subsidence outpacing sedimentation, and detailed subregional-scale sedimentological studies that suggest much greater complexity of sedimentary processes, with gravity flows forming a prominent part of the basin fill (Surlyk, 1984; Ferentinos et al., 1988; Papatheodorou and Ferentinos, 1993; Leppard and Gawthorpe, 2006). In this paper, we present results of a detailed (1 cm scale) lithofacies study of the El Qaa half graben (Moustafa, 1987, 1993; Sharp et al., 2000a, 2000b), an Oligocene–Miocene rift-margin basin on the eastern Suez Rift flank of the Sinai Peninsula, Egypt (Fig. 1). The purpose of our study was to use exceptionally well-exposed outcrops to quantify synrift slope and base-of-slope strata, allowing us to reconstruct paleoflow processes and pathways within a rift-margin basin. These data along with compiled published subaqueous rift-margin basin case studies are compared and allow us to speculate on general controls and predictable trends within analogous riftborder settings. GEOLOGICAL SETTING The Oligocene–Miocene intracontinental Suez Rift was a fault-bounded marine trough (Fig. 1). The eastern rift margin is divided into

GSA Bulletin; January/February 2013; v. 125; no. 1/2; p. 109–127; doi: 10.1130/B30665.1; 13 figures; 1 table.

For permission to copy, contact [email protected] © 2013 Geological Society of America

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A

Med. Sea

B

Postrift

29°00′

10 km

N

Synrift

Study area

23.5 Ma Gulf of Suez

Sinai

Baba-Sidri Fault

Nezzazat Fault

Prerift Study area 100 km

El Qaa Fault block

N Red Sea 33°10′ Legend:

Normal Fault

Syncline

Anticline

200 m

Figure 1. Geographic location and regional tectonic setting of the eastern Suez Rift border fault system. (A) Regional map showing study area location. (B) Simplified geologic map and simplified stratigraphic column of the El Qaa fault block modified from Moustafa (1987, 1993); Sharp et al. (2000a, 2000b); and Khalil and McClay (2001).

three distinct fault block provinces (Moustafa, 1993). The El Qaa fault and half graben form the central province, associated with steep, west-dipping faults (Moustafa, 1976). The El Qaa half graben is oriented NNW-SSE, and it is 12–20 km wide and 40 km long. It narrows to the north, where it terminates north of Wadi Baba, the location of this study (Moustafa, 1993; Gupta et al., 1999; Fig. 2). Large-scale structures of north Wadi Baba from east to west include: (1) the rift-bounding, Baba-Sidri (normal) fault, with throws of 2–3.5 km down to the SW, which juxtaposes Precambrian–Cretaceous prerift units against Miocene synrift units; (2) the asymmetric El Qaa syncline, with a steep northeastern limb and gently inclined southwestern limb; (3) the West Baba antithetic (normal) fault, which is downthrown to the E; and (4) the Nezzazat (normal) fault, throwing 3.5–5 km down to the WNW

(Moustafa, 1987, 1993; Gawthorpe et al., 1997; Sharp et al., 2000b; Khalil and McClay, 2001; Figs. 1 and 2A; Table 1). Miocene synrift sedimentary rocks of the Gharandal Group unconformably overlie Cretaceous to Paleogene prerift strata (Fig. 2; Table 1), and at north Wadi Baba, these rocks comprise the terrestrial, marginal marine, and marine Nukhul Formation, and the marine Rudeis Formation (Robson, 1971; Garfunkel and Bartov, 1977). Regional tectono-stratigraphic models, inclusive of Wadi Baba, envisage a marine transgression during Nukhul to Rudeis Formation times linked to increasing fault displacement (Moustafa, 1987, 1993; Sharp et al., 2000a, 2000b). Sharp et al. (2000b) used 1–10-km-scale field relationships (as revealed by mapping) to infer hemipelagic, turbidite and localized Gilbert delta deposition during Rudeis Formation times.

DATA AND METHODS This study uses outcrop data to understand the sedimentary processes and depositional history of the Rudeis Formation in north Wadi Baba. Sedimentary logging (centimeter scale) and geologic mapping (1:10,000 scale) facilitated interpretations of sedimentary processes and patterns, and structural field relationships (Figs. 2 and 3). RESULTS Stratigraphy and Sedimentary Process The Rudeis Formation is >360 m thick, bounded at its base by heavily bioturbated, condensed, sheet Nukhul Formation beds (Malpas et al., 2005), and at its top by contemporary badland erosion. Two informal stratigraphic units—

TABLE 1. PRERIFT AND SYNRIFT ROCK TYPES FROM REGIONAL SUEZ RIFT STUDIES* Thickness Formation (m) Lithology Rudeis 300–2200 Clastic shales, marls, and sandstone containing foraminifera (Globigerina sp. and Uvigerina sp.). Nukhul 50–100 Clastic sandstones, conglomerates, and shales. Poorly fossiliferous (Miogypsina sp. and Elphidium sp.). Darat 98 Green and brown marls, fossiliferous limestone. Thebes 140–800 Nummulitic limestone, abundant chert nodules and bands, dolomites, anhydrites. Esner 15–40 Marls. Sudr 100–220 Massive white chalk and dark-brown, organic-rich limestone. Matulla 140 Fossiliferous marls (oysters), limestones, chalk, dolomite, chert bands, and sandstones. Wata 150 Macrofossil-rich (ammonites) limestones, dolomites, and marls. Raha 20–80 Glauconitic sandstones, fossiliferous (ammonites, echinoids, lamellibranchiata) limestones, red sandstones, and mudstone. Nubian (Nubia A–D) 600–950 Marine and nonmarine (alluvial) red sandstones, red mudstones, conglomerates, shales, and limestones. Basement Unknown Feldspathic gneisses intruded by unmetamorphosed sills and dikes. *Garfunkel and Bartov (1977); Sellwood and Netherwood (1984); Schutz (1994); El Azabi and El Araby (2007); Samuel et al. (2009).

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Submarine slope processes in rift-margin basins, Miocene Suez Rift, Egypt Legend: Normal Fault

Reverse Fault

Syncline

Anticline

Unconformity

Outcrop outline

A

N

Baba-Sidri Fault

B Era

Nezzazat Fault

System

Series

QUATERNARY

HolocenePliocene

Group

Formation

Informal subdivisions

Suez Rift phase

Wadi Gravels

Postrift

R2

CENOZOIC

Rudeis Fm. NEOGENE

Miocene

Gharandal Group Nukhul Fm.

West Baba Antithetic Fault

R1 IV III II I

Synrift

Darat Fm. Eocene PALEOGENE

El

Q

aa

Thebes Fm. Esner Fm.

Paleocene

Sy

nc

lin

Tr ac

Sudr Fm.

e MESOZOIC

e

Matulla Fm.

Upper

.

CRETACEOUS

Wata Fm.

Prerift

Raha Fm. Lower Nubia A

ba i Ba Wad 0

SW West Baba Antithetic Fault

0.5

1 km

Baba-Sidri Fault

NE

El Qaa Syncline

Nezzazat Fault 0

PRECAMBRIAN PALEOZOIC

JURASSIC PERMO-TRIASSIC CARBONIFEROUS

Nubian Sandstones

DEVONIANCAMBRIAN

Nubia B Nubia C + D

Basement

200 400

Figure 2. (A) Simplified geologic map (modified from Sharp et al., 2000b) and cross section of the study area. (B) Composite north Wadi Baba stratigraphy (modified from Moustafa, 1987; Schutz, 1994; Sharp et al., 2000b).

R1 and R2—are distinguished (Fig. 3). The boundary between R1 and R2 follows Sharp et al. (2000b), who recognized a fine-grained base to R2 in the southern part of the study area. Sedimentary processes are interpreted from six Rudeis Formation lithofacies. Lithofacies discriminations are based on lithology and grain size (Figs. 4 and 5). Lithofacies 1 Calcarenite sandstone is the most common rock type, comprising 54% by thickness of the measured sections (Fig. 4D). Calcarenites are fine- to coarse-grained sandstones that are poorly to well sorted (Fig. 4). They contain lithic grains of prerift Raha to Darat Formation rocks (Table 1), including chert, limestone, sandstone, and mudstone, together with fragmented Miocene body fossils (sp. Miogypsina

foraminifera) that are cemented by calcium carbonate. Bed thicknesses range from 0.02 m to 1 m. The beds are dominated by concave-up erosively based lenticular geometries, which pinch out over 2–250 m (Figs. 5 and 6C). Subordinate flat-lying conformably based tabular beds that extend across the study area were also observed (Figs. 4–6). Upper bedding planes are sharp and flat-lying or wavy. The majority of beds are nongraded, but inverse, normal, and more complex grading arrangements are present (Figs. 4A–4C). Sedimentary structures include planar ripple cross-beds, parallel laminae, convolutions, load casts, groove casts, and imbricated and aligned clasts (Fig. 4). Massive (structureless) beds are also common (Figs. 4A–4C). Seven bed types are distinguished by vertical arrangement of sedimentary structures, these include: (1) mas-

sive beds; (2) massive overlain by parallel laminations and rippled beds; (3) massive overlain by rippled beds; (4) parallel laminations overlain by massive beds; (5) massive overlain by ripples and massive beds; (6) parallel lamination beds; and (7) rippled beds. Bioturbation is widespread and most intense within the upper parts of beds (Figs. 4A–4C). Trace fossils include Thalassinoides and Skolithos, together with an array of indistinct burrows. Fossil fragments include corals, shells, oysters, rhodoliths, Miogypsina, echinoids, and plant material. Fossil fragments are distributed throughout beds or in discrete parallel laminae (Figs. 4A–4C). Carbonaceous plant matter may form upper bed caps. Interpretation The lithic grains and fossil fragments of calcarenite sandstones were derived from prerift units (Table 1; Fig. 2B) and contemporaneous


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360

340

20

A

Silt Fine sand Medium sand Coarse sand Granule Pebble Cobble Boulder Block

Composite log of Northern end of outcrop

Height in Section (m)

Strachan et al.

Log 3

N

Z FS MSCS G P C Bo Bl

Log 1 Log 4 Log 5

320

Log Log 7 6 Log Log 9

Log 2

Log 10

Log 12

8

300

300

1 km

Grain size

Composite log of Southern end of outcrop

320

0.5

0

R2 280

280

R1

Nukhul Fmn

240

240

220

220

Log 12

IV III II I

260

260

Log 11

Log 2

R2 Rudeis Fmn

Height in section (m)

Height in section (m)

Flutes & cross-beds

200

180

Grooves

R2

200

n=13

180

n=3 Grooves

Flutes & cross-beds

160

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120

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n=20

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0 SW NE-SW transect of 12 logs not to scale Percentage mudstone Average grain size

Average grain size (mm)

100

NE

Figure 3. (A) Composite sedimentary logs from northern (logs 1 and 5) and southern (logs 2, 11, and 12) ends of the outcrop. Informal stratigraphic units R1 and R2 are highlighted. Simplified geological map shows log locations. Summary rose diagrams show paleocurrents from stratigraphic units R1 and R2. (B) Average (mean) grain size and mudstone content statistics from each log. Numbers correspond to logs.

Miocene reefs. The range of sorting implies variable textural maturities, with both immature and mature grain populations. Such mixed populations suggest that sediment particles underwent varied amounts of reworking, sorting, and transport prior to being redeposited by submarine unidirectional flows.

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Abundant erosionally based, lenticular beds suggest that unidirectional flows were highly erosive and able to scour the substrate. Several processes result in scoured seafloor depressions, including: (1) allogenic megaflute and flute formation (e.g., Elliott, 2000; Kane et al., 2009), (2) slump scar formation (Locat

and Lee, 2002), and (3) erosional turbidite channel formation (e.g., Kneller, 2003; Pyles et al., 2010). Lenticular beds 6 m (Figs. 5G–5I). Beds and laminae appear tabular; however, they are frequently truncated by erosive overlying sandstones and conglomerates (lithofacies 1–2, 4–6) that prevent lateral correlation of mudstones. Mudstones are occasionally deformed by folding and faulting (Fig. 5I). Bed and lamination boundaries are sharp or gradational, and generally conformable and flat lying (Figs. 4A–4C). Beds are typically nongraded, but inverse and more complex vertical grading is observed. Parallel laminae, siltstone lenses, and massive beds were observed (Figs. 4A–4C). Satin spar fibrous gypsum bands are commonly interbedded with mudstones and can form undeformed laminae and beds, folded intervals, and anastamosing branching networks that crosscut bedding (Figs. 5G–5I). Bioturbation was rarely observed, but where present, Chondrites traces were identified. Interpretation Mudstone deposition is indicative of lowenergy deposition devoid of coarse-grained input. Thin beds and laminae suggest slow or


L

K

J

LITHOFACIES 4

0.1 m Amalgamated & crosscutting beds

0.3 m

0.3 m

LITHOFACIES 5

Matrix-supported

Matrix-supported

Lenticular conglomerate beds

Clast-supported

Q

P LITHOFACIES 6

O

N

LIMESTONE MONOMICT CONGLOMERATE

M 15 m

20 m

0.4 m Chert conglomerate

R

Load and flame structures

Sandstone boulders Limestone clasts

5m Folded interval

POLYMICT CONGLOMERATE

Laterally extensive unit

CHERT-DOMINATED OLIGOMICT CONGLOMERATE

Submarine slope processes in rift-margin basins, Miocene Suez Rift, Egypt

Laterally extensive, poorly sorted bed

Figure 5 (continued ). (J–L) lithofacies 4, chert-dominated oligomict conglomerate; (M–O) lithofacies 5, limestone monomict conglomerate; and (P–R) lithofacies 6, polymict conglomerate.

dilute deposition, and cumulative mudstone thicknesses of >6 m suggest extended periods of hemipelagic settling or localized ponding of muddy gravity flows. The lack of lateral bed continuity implies either erosion by later gravity flows or that deposition was localized. Some mud deposition was associated with slopes that later slumped. Nongraded, laminated beds resulted from hemipelagic deposition, while inversely graded beds suggest deposition from unidirectional mud-rich turbidity currents. The association between gypsum and mudstones resulted from diagenetic gypsum formation that displaced host mudstones. The scarcity of trace fossils suggests that conditions were generally unfavorable for burrowing organisms. However, at times, Chondrites ichnofauna suggests lowoxygen conditions. Lithofacies 4 Chert-dominated oligomict conglomerates are the most common conglomerate type encountered, forming 12% of the succession by thickness (Fig. 4D). They include well-sorted,

well-rounded, pebble- to boulder-sized prerift (Thebes Formation) chert clasts and subordinate poorly sorted, angular prerift limestone clasts and marine Miocene fossil fragments. Conglomerates can be matrix-supported, clast-supported, or range from clast- to matrix-supported conglomerates up bed (Figs. 4A–4C, 5J–5L, 6B, and 6C). Matrix ranges from mudstone to very coarse sandstone. Beds are 5 cm to 15 m thick and often lenticular, pinching out over 10–2000 m (Figs. 4, 6B, and 6C). Minor tabular or folded beds are present. Bed bases are gently curved and truncate underlying strata. Upper bedding planes are flat lying (Figs. 4A–4C and 5J–5L). Nongraded, massive beds are common; infrequent normal, reverse, and more complex vertical grading is also observed (Figs. 4A–4C). Chert-dominated conglomerates can display inclined clast imbrication, locally developed flat-lying bedding, steeply dipping bedding planes, and amalgamation surfaces (Figs. 4A–4C, 5J–5L, 6B, and 6C). Highly irregular lenses (10 m wide, up to 3 m high) of conglomerate, containing inclined beds, are also observed on laterally extensive

thin beds (Figs. 6B and 6C). Fragmented fossils, including corals and oyster shells, are observed. Maximum clast size (MPS) and bed thickness (BTh) plots (after Bluck, 1967) reveal that there is no relationship between MPS and BTh and that thicker beds have a smaller range of maximum clast sizes (Figs. 7A–7C). Interpretation Well-sorted texturally mature chert clasts dominate lithofacies 4 and suggest that sediment source areas were rich in well-sorted cherts, and that texturally immature limestone and contemporaneous Miocene fossils were entrained into flows during redeposition. Erosively based, imbricated, massive, clast- and matrix-supported conglomerates are interpreted as the deposits of unidirectional submarine flows. The lenticular nature of most beds suggests that flows were confined within scoured surfaces. The large range in scour widths from 10 to 2000 m implies a range of scales of erosion. We suggest that some surfaces were scoured by erosive precursor flows that bypassed to deposit material further downstream, while other flows scoured


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A Lithofacies 1 Lithofacies 6

Sketch Log

2m

Truncation and incision

Large angular to subangular clasts

W

E

B West Baba Antithetic Fault

Extreme lenticularity

Inclined bedding 4m Lithofacies 4

Lithofacies 1 Log 8

W

Baba-Sidri Fault

E

Log 9

Laterally extensive lithofacies 6 C Local lenticularity

Lenticular beds 30 m

Log 10 Lithofacies 4 Bedded Lithofacies 1 Incisional horizon Log 2 NW

SE

Figure 6. (A) Truncation and incision of calcarenites (lithofacies 1) by chaotic, lenticular polymict conglomerates (lithofacies 6). (B) Extreme lenticularity of chert-dominated oligomict conglomerate (lithofacies 4) associated with the West Baba antithetic fault. (C) Overview showing bed geometries of calcarenites, chert, and polymict conglomerates. Conglomerates are highly erosive and exhibit irregular bed thicknesses. Calcarenites are lenticular and erosive. Local development of a chert conglomerate with inclined bedding is interpreted as a within-channel lateral accretion package.

their own bases and were also contained within them. Bedding and amalgamation surfaces also imply localized erosion of channelized flows. Several localized sediment accumulations of inclined anomalously thick beds (Figs. 6B and 6C) are tentatively interpreted as turbidite channel bars or lateral accretion packages (Abreu et al., 2003; Hesse et al., 2001). Such structures,

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along with amalgamation surfaces, imply protracted periods of channelized flow, where sediment bypass, deposition, and migration from multiple gravity flows resulted in preserved composite deposits. The scarcity of tabular beds suggests that unconfined flows were atypical. Rare folded conglomerate beds are interpreted as submarine slumps.

The variety of graded beds and matrix types suggests that several flow rheologies and mechanisms deposited chert-rich conglomerates, including: (1) cohesive and noncohesive debris flows that deposited nongraded matrix- and clast-supported beds with mudstone and sandstone matrices; (2) waning high-concentration turbidity currents that deposited normally graded matrix-supported beds (R3S3 beds; Lowe, 1982); (3) waxing high-concentration turbidity currents that deposited inversely graded matrixsupported beds (S3R3 beds; Lowe, 1982); and (4) flows transitioning from debris flows to turbidity currents with time (R3S3 overlain by Tb; Bouma, 1962; Lowe, 1982). Maximum clast size (MPS) and bed thickness (BTh) plots have been used to demonstrate the link between flow thickness and competence, which, in cohesive debrites, results in linear trends between MPS and BTh (Bluck, 1967; Nemec and Steel, 1984). Here, there is no relationship between MPS and BTh (Figs. 7A and 7B). We suggest that the absence of any relationship results from: (1) the range of gravityflow processes and deposition modes; (2) the narrow range of clast sizes available to flows (i.e., presorting); and (3) the presence of amalgamated beds. The chert-dominated conglomerates do not preserve evidence of shallow-water or stormwave reworking. This, together with implied gravity-flows processes, suggests deposition on a submarine slope or base of slope. Fossil fragments, however, imply that flows initiated in shallower waters, where they eroded shallowwater detritus, sweeping it downslope in energetic redeposition events. Lithofacies 5 Carbonate oligomict conglomerates are rare, comprising 5% of the succession by thickness. They are composed of pebble- to boulder-sized prerift limestone clasts (Table 1; Figs. 4A–5C and 5M–5O) that are poorly to moderately sorted, occasionally bored, and angular to subangular, together with fragmented and intact Miocene shells, corals, and rhodoliths. Carbonate conglomerates can be matrix-supported, clast-supported, or both matrix- and clastsupported conglomerates within the same bed (Figs. 5N and 5O). Variable matrix types range from mudstones to granule. Beds are 10 cm to 4.5 m thick. Bed geometries include 100–1000-m-wide lenticular beds, tabular sheets, and laterally de-amalgamated beds (Figs. 4A–4C and 5M). Bed bases are sharp, and can be conformable and flat lying, or truncating and concave up (Figs. 4A–4C, 5M, and 5N). Upper bed contacts are flat lying


Submarine slope processes in rift-margin basins, Miocene Suez Rift, Egypt

Maximum Clast (MPS) (m)

0.5 0.4 0.3 0.2

1

0.1

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0

0.7

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6

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8 10 Bed thickness (BTh) (m)

12

16

14

0.01 0.1 1

D

Lithofacies 5

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Power trend

0.1

N= 39 Y = 0.2232x0.3284 2 R = 0.0979

0.1 0.01 0.1

2

3

2.5 BTh (m)

3.5

4.5

4

30

G

Lithofacies 6

MPS (m)

Linear trend

Power trend

10 0.1

N= 57 Y = 0.3191x1.0314 R2= 0.6575

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30

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10

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N= 57 Y= 0.8018x - 0.3723 R2= 0.4975 Y/X= 0.71 R= 0.64972

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4

10

1 BTh (m)

25

15

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1

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0 1.5

Maximum

5 Thickness or size (m)

N= 39 Y= 0.033x + 0.2291 R2= 0.0398 Y/X= 0.2621 R= 0.1996

MPS (m)

MPS (m)

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0

E

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0.5

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0.6

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Maximum C Clast size Bed thickness

4

N= 122 Y = 0.1707x0.2033 2 R = 0.1073

0.1 0

16

Power trend B Thickness or size (m)

A N= 122 Y= 0.0076x + 0.1778 2 R = 0.0146 Y/X= 0.05 R= 0.12096

Lithofacies 4 0.6

MPS (m)

0.7

Maximum

Mean

Minimum

Figure 7. (A–B, D–E, G–H) Bed thickness (BTh) versus maximum clast (MPS) plots for lithofacies 4, 5, and 6 plotted as linear and log-log graphs. (C, F, I) Histograms showing the maximum, mean, and minimum values of bed thickness and clast sizes for lithofacies 4, 5, and 6.

and conformable. Most beds are nongraded, but normal, inverse, and more complex vertical grading is observed (Figs. 4A–4C). Groove casts are present, and internally structureless beds are common (Figs. 4A–4C). Lateral variations in grain size are observed within single beds from conglomerate to pebbly sandstone. The upper parts of several beds contain horizontal burrows (Figs. 4A–4C). MPS and BTh plots show no relationship between MPS and BTh (Figs. 7D–7F). Interpretation The provenance of carbonate conglomerates suggests a sediment source area rich in limestone and contemporaneous bioclastic material. Poor sediment sorting and clast angularity suggest minimal reworking and short transport distances. However, bored limestone clasts allude to a greater longevity of transport for some clasts. Sharp bed-bounding surfaces and lack of reworking suggest deposition from highly energetic unidirectional gravity flows capable of eroding and transporting clasts up to 3 m in diameter. Despite the apparent monomict nature

of beds, flows may have encompassed several prerift limestone types, indistinguishable in hand specimen (Table 1). Variable bed geometries are interpreted to preserve a range of flow behaviors, including confined erosive then depositional, channelized flows (scoured lenticular beds) and unconfined depositional flows (flat-lying conformal sheets). Where stacked beds and de-amalgamation surfaces are encountered, repetition of events focused within the same conduit is implied. Dominant nongraded, structureless beds suggest the majority of events were debris flows. Mudstone to granule matrix suggests deposition of both cohesive and noncohesive debris flows. Infrequent normal, inverse, and complexly graded beds are attributed to high-concentration turbidity currents that waned, waxed, and had more complex flow unsteadiness in time (Lowe, 1982; Dasgupta, 2003). The inclusion of Miocene fossils suggests that flows entrained shallow-marine and reefal fauna. After deposition, colonizing fauna were able to burrow into the upper part of beds, implying relative quiescence between events.

Notwithstanding the small data set, MPS and BTh plots show a lack of correlation between maximum clast size and bed thickness (Figs. 7D and 7E). This is attributed to the range of active flow processes that lack the requisite rheological and depositional mode properties to produce a linear relationship between flow thickness and maximum clast size (Bluck, 1967). Very coarse boulder-size clasts (Blair and McPherson, 1999) indicate deposition from highvelocity and high-strength flows. We suggest that steep slopes would be required to mobilize such material and thus assert the environment of deposition was a submarine slope or basin floor. Lithofacies 6 Polymict conglomerates comprise 8% of the succession by thickness (Fig. 4D). They are poorly to moderately sorted, with a chaotic assemblage of angular to subangular prerift clasts, including limestones, sandstones, mudstones, basalt, and prerift rounded cherts (Table 1). Miocene fossil fragments are also present, including corals, shell fragments, oysters, and


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Strachan et al. rhodoliths (Fig. 4B). Clasts are 1 mm to 25 m in diameter (Figs. 4A–4C, 5P–5R, and 6A). The conglomerates are typically matrix-supported conglomerates, but local clast-supported textures are observed. Matrix ranges from fine to coarse sandstone. Beds are between 10 cm and 25 m thick, and tabular to highly lenticular, pinching out over tens to hundreds of meters (Fig. 6A). Lower bed boundaries are sharp and incisional, cutting downward by >10 m (Fig. 6A), or conformable and flat lying (Figs. 5P–5R, 6A, and 6C). Upper bed contacts are sharp. The majority of beds are nongraded beds, but inverse, normal, and more complex graded examples are also observed (Figs. 4A–4C). Beds are generally structureless, but load casts, parallel laminations, bedding, and folding are observed (Figs. 5P–5R). Bioturbation was not observed. MPS and BTh plots show a wide range of maximum clast sizes and bed thicknesses and a positive relationship between bed thickness and maximum clast size that best fits a power-law trend (R2 value of 0.65; Figs. 7G–7H). Interpretation The polymict nature of lithofacies 6 conglomerates suggests that the sediment source area contained a highly heterogeneous assemblage of clast types and Miocene fauna. Clast size, angularity, and poor sorting further imply textural immaturity, with contained clasts having undergone minimal transport prior to deposition. The dominance of erosively based, matrix-supported beds with boulder-sized clasts suggests deposition from high-energy gravity flows. Bed geometries reveal deposition of highly erosive flows that removed >10 m of the substrate and were therefore contained within their scours, and depositional unconfined flows that deposited sheets. Nongraded, massive beds dominate and were deposited by cohesive debris flows with minor noncohesive debris flow (Mulder and Alexander, 2001) deposition. Additionally, rare inverse, normal, and more complex graded beds with tractional sedimentary structures imply infrequent deposition from high-concentration turbidity currents (Lowe, 1982). A steep submarine slope is considered necessary to have transported the contained clasts. The positive power-law correlations between MPS and BTh suggest that flow thickness and competence are linked (Bluck, 1967; Nemec and Steel, 1984). This further implies that the principal depositional process was via en masse freezing of cohesive debris flows (Bluck, 1967; Nemec and Steel, 1984). The statistical power distribution suggests that larger clasts are more readily entrained by large flows, with positive feedbacks between erosion, acceleration, and

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competence. The R2 value of 0.65 (Fig. 7H) is interpreted to result from a range of viscosities, and related changes in flow processes, associated with individual events. Lithofacies Distributions, Grain-Size Variation, and Paleocurrents The lateral and vertical distribution of lithofacies 1–6 within the measured section is highly heterogeneous (Figs. 4 and 8). The dominance of laterally discontinuous, lenticular beds and highly heterogeneous vertical stacking impedes detailed lateral correlations and thus hinders traditional facies association and succession analyses (Fig. 8). Instead, general observations are offered on grain-size variations, degree of lithofacies heterogeneity, paleocurrents, and mudstone content. The studied succession contains a positively skewed distribution of grain sizes with a mean of 18.8 mm (coarse pebble), median of 0.28 mm (medium sand), and mode of 0.188 m (fine sand). Clasts exhibit both immature and mature textures indicative of a range of provenances that have undergone differing amounts of reworking prior to redeposition. Abundant angular and subangular clasts indicate minimal transport and short-lived flows. Average grain sizes for each sedimentary log reveal a broad fining from NW to SE (Fig. 3B) with logs 6–7 containing the finest-grained average (Fig. 3B). This reflects the fact that logs 6–7 sample the lowest parts of R1, which are finer-grained than the upper part (Figs. 3B and 9). Overview graphic logs of stratigraphic units R1 and R2 show two broad coarsening-up cycles (Fig. 3A). However, detailed grain-size plots at the outcrop scale (>360 m) reveal that a coarsening-up trend is only present in R1, while R2 displays a vertically unordered grain-size distribution (Fig. 9). The reason for this apparent discrepancy is given by one or two thick, coarse beds that bias graphic sedimentary logs (e.g., Hiscott, 1981; Fig. 3). Coarsening-up patterns are, however, commonly observed at the 360 m) scale are unordered in both R1 and R2 (Fig. 9), which is unsurprising given the dominance of lenticular bedding. The three graphic sedimentary logs in Figure 8 demonstrate the vertical distribution of lithofacies. Logs are hung on a tabular lithofacies 6 unit that occurs near the top of R1 (Fig. 8). Greater lithofacies heterogeneity occurs in the NW and central parts of the study area (Fig. 8). Sandstones and mudstones (lithofacies 1–3) dominate the SE part of the study area, where fewer lithofacies types occur. Mudstone content

increases toward the SE and is generally below 15% (Figs. 3B and 8). Paleoflow directions in R1 reveal a polymodal distribution with dominant flows toward the SW and NW, and minor transport toward the NE and SE (Fig. 3A). Paleocurrent directions in R2 are more unimodal, with a dominance of westward-directed currents; however, it should be noted that only small numbers of directions were measured (Fig. 3A). Interpretation The heterogeneous lithofacies organization indicates a disorganized sediment dispersal system dominated by episodic depositional events. The coarse-grained, positively skewed character of the sediments suggests a spectrum of highly energetic, short-duration gravity flows (Walker, 1975; Lowe, 1982) that poured down steep submarine terrain. Deposition is inferred to be relatively proximal throughout the study area with prerift clasts and Miocene contemporaneous reef material originating from an adjacent margin. Paleocurrent directions suggest that flows were mainly from the eastern margin, adjacent to the Baba Sidri fault (Fig. 3A). The western hanging-wall crest is considered a minor sediment source area, because an almost complete prerift succession is preserved, implying early burial by synrift sediments (Fig. 2). Fining toward the SE suggests that there was a smaller flux of coarse-grained material reaching this area, and thus a greater preservation potential of fine-grained deposits because fewer erosive flows occurred there. The broad coarsening-up pattern in stratigraphic unit R1 suggests that with time the slope prograded basinward, flow magnitudes increased, the availability of large prerift clasts increased, and slopes steepened. We speculate that earthquake-induced shaking, catastrophic river flooding, and storm-induced remobilization of shallow-marine (shoreface and shelf) sediments might have triggered the gravity flows. The tabular lithofacies 6 bed that occurs at the top of R1 is interpreted to have formed by collapse of the eastern margin (Fig. 9). The base of stratigraphic unit R2 marks a reduction in conglomerate grain size during earliest R2 times (Figs. 3A and 9). This is speculatively attributed to a reduction in fault activity, leading to a reduction of paleo-earthquakes and relative sealevel rise. Stratigraphic unit R2 coarsening- and thickening-up patterns are highly disorganized, suggestive of a complex depositional system with localized deposition and erosion linked to allocyclic processes. Sharp et al. (2000b) identified a series of prograding deltaic clinoforms from photographs of inaccessible locations positioned


Submarine slope processes in rift-margin basins, Miocene Suez Rift, Egypt CU/FU Patterns

Pie chart key

Key

Lithofacies 1 2 3 45 6

Lithology Mudstone

180

Sandstone

Lithofacies 1

Lithofacies 3

Lithofacies 5

Conglomerate

Lithofacies 2

Lithofacies 4

Lithofacies 6

n= 223

R2

n= 204 160 CU/FU Patterns


140


R2

180

CU/FU Patterns 160

160

120

140

140

100

R1

120

80

Logs aligned on laterally extensive conglomerate

120 100

100 R1

60

80

80

40

R1

60

60

20

Height (m)

40

40 20

Cs Pe Bo Bl Log 3

n= 163 NW

Height (m)

Stratigraphic units 20

Height (m)

Z

Z

Cs Pe Bo Bl Log 12

Z

Cs Pe Bo Bl Log 10

Stratigraphic units

Stratigraphic units SE

Figure 8. Summary sedimentary logs and lithofacies charts from northern, central, and southern parts of the study area (logs 3, 10 and 12; Fig. 3). Logs are hung from a rare tabular lithofacies 6 bed. Pie charts correspond to data within each log and show number of beds by lithofacies. N = number of beds. CU/FU—Coarsening up or fining up. For grain-size abbreviations, see Figure 3.

above the studied succession of this study. These clinoforms demark narrow deltas ( FW thickness

Nukhul Formation (unit 3)

Log 8 Footwall

Inclined bedding

Log 9

10m Hanging wall

Local thickening into hanging wall

NW

0

SE

Eroded fault plane

Log 1 Ru

de

is F

orm

on ati

un

it R

Basement Baba-Sidri Fault

Sudr Formation

2

Matulla Formation

Thebes Formation

Rude

1 km

B

Prerift synrift unconformity - karst surface (Thebes Fmn) overlain by upper R1 strata

Extensive collapse unit Lithofacies 6

0.5

is Fo

Drag folding

rmati

on u

nit R

Overturned footwall

Overturned hanging wall

1 Fault 2 (F2)

100m

Fault 1 (F1)

SW

NE

Figure 10. (A) Oblique view of West Baba antithetic fault. HW—hanging wall; FW—footwall. (B) Oblique view of synthetic faults and overlying Rudeis Formation units. Note the diachronous nature of prerift-synrift unconformity. (C) Simplified geologic map showing the location of photos A and B (see Fig. 2 for key).

DISCUSSION Sedimentary Model of Evolution Rudeis Formation Unit 1 (R1) Deposition initiated with a spectrum of gravity flows, including erosive channelized silty, sandy, and pebbly turbidity currents; unconfined sandy and silty turbidity currents; and numerous bypassing flows (Fig. 11A). Hemipelagic mud deposition occurred at times of minimal coarse-grained sediment input and may reflect relative sea-level rise. Turbidity current channels were deflected against faulted seafloor topography, leading to polymodal paleocurrents (Figs. 3B and 11A). The system aggraded and perhaps prograded with time, forming a com-

plex network of heterogeneous, coalescing lenticular and minor tabular turbidites, debrites, and slumps. Myriad channelized flows, with a range of paleoflow directions, additionally suggest multiple point sources, while extensive tabular collapse units imply a line source along the length of the eastern margin. Distinct conglomerate types with discrete provenances also suggest that a range of terrestrial drainages were located in the northern and central parts of the study area, shedding prerift clasts and Miocene reef detritus offshore from the eastern margin, with a possible minor western sediment source during earliest time (Fig. 11A). The variety of textural maturities in prerift clasts indicates erosion and transport through a range of terrestrial environments, including rivers and alluvial fans

for texturally mature clasts, and cliffs and hillslopes for immature clasts. Our study recorded little evidence of in situ shelf sediment, although Sharp et al. (2000b) inferred several small deltas adjacent to the Baba-Sidri fault system. Sediment staging areas include river mouths, small fan deltas (Sharp et al., 2000b), hillslopes, cliffs, and narrow transient shelves (here we assume shelf widths of 100 m), on submarine slope and basin floor environments. Several early active normal faults breached the seafloor,


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Strachan et al. Figure 11. (A) Schematic diagrams showing gravity-flow pathways and bathymetry evolution from unit R1 (divided into early to mid, and latest stages). (B) Schematic diagrams showing gravity-flow pathways and bathymetry evolution from unit R2 (divided into early and late stages). Small patch reefs are included in several cartoons; their positions are very speculative. The cartoons depicted here show the structural reinterpretation described in this paper. We recognize that the data may also be interpreted as a faulted monocline (for faulted monocline cross sections, see Sharp et al., 2000a, 2000b).

R1 - Early to mid

Sea level

Small reef at river mouth

A N

Multidirectional gravity flows

1 km Baba-Sidri Fault

F2

F1 West Baba Antithetic Fault R1- Latest

creating steep, stepped fault-scarp slopes and local hanging-wall depocenters (Fig. 11A). The relief of these structures affected gravity-flow dynamics and deposition through the formation of above-grade incisional channels (Kneller, 2003; Samuel et al., 2003) and deflected flows (Edwards et al., 1994). West Baba antithetic and F1 fault movement continued throughout much of R1 times, rotating the seafloor, creating localized sites of subsidence, generating angular unconformities, and deforming coeval sediments via drag folding (Figs. 10 and 11A). The West Baba antithetic and F1 faults became inactive in later R1 times and were buried by widespread, nonfaulted deposits that formed by collapse of the eastern margin (Fig. 11A). We suggest that the heterogeneous distribution of facies was controlled by: (1) autocyclic slope-channel processes, such as avulsion, lateral migration, and channel breaching; (2) multiple terrestrial drainages; (3) myriad gravity-flow types; (4) tectonically driven uplift, subsidence, and surface tilting; and (5) multiple gravity-flow triggers. The increasing abundance of mediumcoarse block (sensu Blair and McPherson, 1999) conglomerates suggests that catastrophic collapse of landslide-prone slopes by earthquake shaking became more frequent with time. Controls on submarine slope aggradation and progradation may have been controlled by increasing earthquake recurrence rates or relative sea-level fall. Evidence of enhanced subsidence leading to sediment starvation and mudstone deposition (Prosser, 1993) during this time is absent and is instead completely outpaced by high sedimentation rates. Water depths remained below storm wave base throughout. Rudeis Formation Unit 2 (R2) Westward-directed gravity flows accompanied widespread turbidite sand and minor hemipelagic mud deposition in early R2 times (Fig. 11B). A succession of increasingly large-

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Large-scale collapse of eastern margin

Uplift

Dominantly westerly directed flows Subsidence Rudeis Formation sediments

Prerift-synrift unconformities

Baba-Sidri Fault West Baba Antithetic and F1 faults are inactive F2

R2 - Early

B

Westerly directed gravity flows Uplift

Subsidence Rudeis Formation sediments

Baba-Sidri Fault Drag folding by ongoing F2 movement F2

R2 - Late

River incision Relative sealevel fall

Delta progradation

Westerly directed gravity flows

Subsidence

Rudeis Formation sediments


F2 is inactive

Baba-Sidri Fault

Uplift & rotation of synrift strata

Submarine slope processes in rift-margin basins, Miocene Suez Rift, Egypt magnitude erosional gravity flows (including turbidity currents, debris flows, cogenetic flows and slumps) originating from the eastern margin followed (Fig. 3). Beds are composed of medium sand– to boulder-sized prerift clasts and Miocene reef fragments possibly derived from river mouths, alluvial planes, and deltas on a steep uplifting hinterland. Sharp et al. (2000a, 2000b) interpreted deltaic clinoforms as preserving evidence of shallowing-up trends in latest R2 times. The deltaic clinoforms of Sharp et al. (2000a, 2000b) are interpreted here to have prograded across R1 and early R2 sediments that infilled fault-generated accommodation, generating a narrow (