Depositional processes, triggering mechanisms and sediment composition of carbonate gravity flow deposits: examples from the Late Cretaceous of the south-central Pyrenees, Spain

Sedimentary Geology 146 (2002) 155 – 189 www.elsevier.com/locate/sedgeo

Depositional processes, triggering mechanisms and sediment composition of carbonate gravity f low deposits: examples from the Late Cretaceous of the south-central Pyrenees, Spain Peter A. Drzewiecki*, J. Antonio Simo´1 Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton Street, Madison, WI 53706, USA Received 29 October 1998; accepted 18 June 2001

Abstract Cenomanian through Coniacian strata near the town of Sopeira in the south-central Pyrenees (northern Spain) are composed of a variety of autochthonous and allochthonous carbonate slope lithologies that are divided into six depositional sequences based on facies distribution patterns and stratal relationships. The sequences record three major phases of platform margin evolution: rifting, burial, and exhumation. During the first phase (sequences UK-1, UK-2, UK-3, UK-4, and lower UK-5), deposition occurred on the edge of a wrench basin, and a normal fault located beneath the platform margin strongly influenced slope evolution. Background hemipelagic sediments on the slope were commonly redeposited by submarine slumps and slides. More intense reworking resulted in matrix-supported, slope-derived megaconglomerates (debrites). During the Cenomanian and Turonian, seismically triggered debris flows originated at the platform margin, bypassed the upper slope, and were deposited on the lower slope as polymictic, clast-supported, matrix-rich megabreccias. The megabreccias form channelized and sheet-like bodies with erosional basal surfaces. Shallow carbonate environments backstepped during the Late Turonian and Coniacian, but displacement along the fault at this time resulted in the development of a steep submarine scarp and the exposure of Cenomanian and Lower Turonian strata to submarine erosion. Matrix-poor, margin-derived megabreccias form a thick talus pile at the base of the scarp. Some of the breccias were transported into the basin as debris falls, forming sheet-like beds. Marl eventually buried the Coniacian scarp in sequence UK-5, resulting in the second major phase of platform slope evolution. The slope profile at this time was relatively gentle, and redeposited material is less common. In the third phase (sequence UK-6), tectonically induced bankward erosion during the Santonian resulted in a high (greater than 800 m) erosional scarp with a regional east – west trend that was subsequently onlapped by siliciclastic turbidites. Rejuvenation of erosion in the same vicinity suggests that long-term tectonism controlled the position of the slope, rates of erosion, and sediment type on the slope. Sediment gravity flow processes are laterally and temporally related. Submarine slide and slump deposits commonly grade laterally downslope into slope-derived megaconglomerates. Debris flows that originated at the platform margin appear to have initiated slumps, slides, and other debris flows on the slope. Debris fall deposits are commonly capped by coarse, graded, lithoclastic packstones that may represent turbidites generated by the debris falls. Sediment fabric exerted a profound impact on depositional processes, distribution of facies, and morphology of the slope. Fine-grained, mud-rich, lower slope deposits were unstable at even moderate slope angles, and have been extensively redeposited. Redeposition of grain-rich, upper slope facies

* Corresponding author. Present address: ExxonMobil Upstream Research Company, PO Box 2189, Houston, TX 77252-2189, USA. Tel.: +1-713-431-6377; fax: +1-713-431-6166. E-mail addresses: [email protected] (P.A. Drzewiecki), Simo´@geology.wisc.edu (J.A. Simo´). 1 Tel.: +1-608-262-5987.

0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 0 1 ) 0 0 1 7 1 - 3

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was triggered by syndepositional seismic activity and upslope migration of instability and erosion. In the presence of mud, the transport mechanisms are typically cohesive debris flows, which were able to carry material onto the lower slope and into the basin. When no mud was available, rock falls and debris falls were the dominant sediment gravity flows, and their deposits are restricted to a position on the hanging wall proximal to the fault. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Carbonate platform; Slope; Upper Cretaceous; Spain; Depositional controls

1. Introduction Carbonate slope environments provide critical geological information about both depositional and erosive processes that sculpt carbonate platform margins. Platform margin morphology and evolution are sensitive to changes in environment, physico-chemical oceanic conditions, tectonics, sea level, and biotic composition. Variations in the geometries and lithologies of slope facies reflect changes that occur on the platform margin and shelf. In some regions, breccia clasts found in the slope environment are the only preserved record of high-energy margin or shallow platform facies (e.g. James, 1981). In addition to providing information about the depositional history of a platform margin and upper slope, carbonate breccias, megabreccias, and conglomerates deposited on the slope have been considered important indicators of low relative sea level and have been used to reconstruct eustatic sea level history (Sarg, 1988; Haq et al., 1987). More recently, Hanford and Loucks (1993) recognized the occurrence of megabreccias in all systems tracts of carbonate sequences, but still emphasized their importance in the lowstand systems tract. Finally, megabreccia deposits can form significant hydrocarbon reservoirs (Enos and Moore, 1983; Cook and Mullins, 1983; Enos, 1985). Carbonate slope deposits, including sediment gravity flows, have been the subject of numerous studies (Table 1). These investigations have focused on descriptions and interpretations of depositional processes, and speculated about the triggering mechanisms responsible for the initiation of sediment gravity flows. Triggering mechanisms include eustatic and relative sea level changes, tectonic seismicity, platform oversteepening, differential compaction, and bolide impacts. Slope stability and the effectiveness of various triggering mechanisms is partly dependent on the composition of overall slope deposits (especially the availability of mud) and early diagenetic lithification.

This study uses data from slope deposits of six Upper Cretaceous carbonate sequences (south-central Pyrenees, Spain). It includes a description of slope facies, an evaluation of depositional mechanisms, and an interpretation of possible triggering mechanisms for carbonate megabreccias. In particular, it addresses the relationship between the composition of slope deposits, the evolution of the platform margin, and the tectonic factors that control this evolution. The deposits described in this article are carbonate muddominated, and the importance of mud in controlling the depositional history is evaluated. The results are briefly compared to literature accounts of other carbonate slope environments in order to draw some fundamental conclusions on aspects of carbonate slope deposition.

2. Geological setting The Sopeira region in the south-central Pyrenees, northern Spain, contains Cenomanian through Santonian strata that were deposited on the southern margin of the Pyrenean basin, and are exposed today on the northwestern portion of the Montsec thrust sheet (Fig. 1). The outcrops consist of slope sediments preserved on a continuous, but tectonically deformed exposure about 2.5 km long (Fig. 1). Reconstruction of the slope environment was accomplished by tracing time-significant surfaces between closely spaced measured sections. Lower Cretaceous sediments that underlie the Cenomanian – Santonian strata in this region are composed of coarse-grained, cyclic carbonate packstones to grainstones containing shallow-water biota. The sediments were deposited on a shallow shelf, on the edge of a basin that was located to the east of the study area (Van Hoorn, 1970). In Late Albian time, wrench faulting between the Iberian and European plates resulted in the formation of a deep, narrow

Table 1 Comparison of slope deposits and depositional processes from several platforms containing carbonate breccias Slope description

Major slope lithologies and depositional processes

Breccia triggering mechanism

References

Franklinian Shelf (North Greenland; Cambrian to Early Ordovician)

steep, mud-rich; depositional/ breccia bypass slope

carbonate and clastic mudstones, chert (suspension fall-out, dilute turbidites); turbiditic siltstones and sandstones (turbidites); convoluted slope sediments (slides, creep); slope- and some shelf-derived, clast-supported megabreccia (debris flows, high density turbidites)

Surlyk and Hurst, 1984; Surlyk and Ineson, 1987; Ineson and Surlyk, 1992, 1995

Hales Limestone (Nevada, USA; Cambrian to Early Ordovician)

steep, mud-rich; depositional/ breccia bypass slope

Cow Head Group (Newfoundland, Canada; Cambrian to Early Ordovician)

steep, mud-rich; depositional/ breccia bypass (?) slope

oversteepening of margin and seismic activity

Hubert et al., 1977; Hiscott and James, 1985; James and Stevens, 1986; James et al., 1987, 1989

Ancient Wall and Miette reefs (Alberta, Canada; Late Devonian)

steep, grain-rich; depositional slope

fine-grained limestone (suspension fall-out); deformed finegrained limestone (slumps and slides); shelf-derived calcarenites (tubidites, contourites); slope-derived, clast- or matrix-supported megabreccia (debris flows); margin- and slope-derived, clast- or matrix-supported megabreccia (debris flows) hemipelagic sediments (suspension fall-out); medium to coarse calcarenites (gravity transport from shelf); graded, matrix- and clast-supported conglomerates (turbidites); clast-supported, slope-derived, limestone plate and chip conglomerates (debris flows); clast-supported, slope- and shelf-derived conglomerates (debris flows); clast- to matrix-supported, shelf- and slopederived, megaconglomerates (debris flows) carbonate mudstone (suspension fall-out); allodapic carbonate sands, calcarenites, occasionally graded (turbidites, downslope creep); reef-derived talus blocks (rock falls); reef-derived, occasionally graded breccias (debris flows); reef-derived, matrix-supported megabreccias (debris flows)

seismic activity for slope-derived breccias; erosion during lowstand for marginderived breccias slope instability induced by relative sea level fall

probably seismic activity (tsunamis, differential compaction, or porefluid overpressuring also possible)

Canning Basin (Northwestern Australia; Late Devonian)

steep, grain-rich; depositional slope

skeletal calcarenites and calcirudites (downslope gravity-driven movement, in situ deposition); reef-derived talus blocks (rock falls); reef-derived, clast- and matrix-supported, often reversegraded megabreccias (rock falls, avalanches, debris flows)

Midland Basin (West Texas, USA; Early Permian)

steep, grain-rich; depositional/breccia bypass (?) slope

shales, micrites, dark laminated mudstone (suspension fall-out); fine-grained sandstone (suspension fall-out, turbidites); skeletal, peloidal, oolitic packstone to grainstone (turbidites?); marginderived, clast- and matrix-supported megabreccias (debris flows)

Srivastava et al., 1972; Mountjoy et al., 1972; Cook et al., 1972; Hopkins, 1977; Mountjoy, 1989; Geldsetzer, 1989; McLean and Mountjoy, 1993 Playford and Lowry, 1966; Playford, 1980; Playford et al., 1989 Mazzullo and Reid, 1989; Saller et al., 1989; Leary and Feely, 1991

seismic activity, early cementation, and differential compaction contributed erosion during sea level lowstand (possibly highstand shedding)

Cook and Taylor, 1977; Cook, 1979; Cook et al., 1989

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Name, age, and location

(continued on next page) 157

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Table 1 (continued) Slope description

Major slope lithologies and depositional processes

Breccia triggering mechanism

References

Grayburg Fm. (West Texas, USA; Late Permian) Rader and Lamar formations (West Texas, USA; Late Permian)

steep; erosional slope

shallow platform-derived, clast- and matrix-supported megabreccia (slumps, debris flows)

Crawford, 1981; Franseen et al., 1989

steep, grain-rich; depositional/breccia bypass (?) slope

Dolomites (Northern Italy; Middle to Late Triassic)

steep, grain-rich; depositional slope

fine sandstone, bioturbated limestone, graded limestone, laminated sandstone (suspension fall-out, turbidites, bottom currents); margin- and slope-derived, clast- and matrixsupported conglomerates, megabreccias, and breccias (densitymodified grain flows, debris flows, rock falls ); sandy fusulinid turbidites, inversely-graded skeletal-peloidal wackestone to grainstone (turbidites); ungraded skeletal packstone (debris flows) clast-supported, matrix-poor, margin-derived megabreccias (rock falls, avalanches); graded skeletal grainstone (storm-generated turbidites)

submarine erosion possibly initiated or intensified by sea level fall uncertain (seismic activity, oversteepening during highstand, undercutting, tsunamis are all possible)

Yose, 1991a,b; Masetti et al., 1991; Harris, 1994

Eastern Alps (Switzerland; Early Jurassic)

steep; erosional slope

margin oversteepening during highstand (or erosion during lowstand) seismic activity

Lusitanian Basin (Portugal; Late Jurassic)

gentle, mud-rich; depositional slope

Gorbea Platform (Northwestern Spain; midCretaceous)

steep, grain-rich; depositional/breccia bypass slope

marls, mudstones (suspension fall-out); massive calcarenites (mass gravity flows, modified grainflows); graded calciturbidites (turbidites); laminites (bottom currents); unsorted, clast-supported, matrix-poor, polymictic breccia (rock falls, avalanches); marginderived, clast-supported, matrix-poor megabreccias (debris flows); clast- and matrix-supported polymictic conglomerates (debris flows); matrix-supported, slope-derived pebbly mudstones (debris flows); deformed mudstone beds (slumps) ammonitic lime mudstones (suspension fall-out); thin-bedded turbiditic packstone and grainstone (turbidites); contorted beds (slumps); slope- and margin-derived megabreccias (debris flows, turbidites) upper slope: coral wackestone (in situ mounds); skeletal grainstone; margin-derived talus blocks (rock falls, gravity sliding of individual blocks). Lower slope: marls (suspension fall-out); lutites, sandstones (suspension fall-out, turbidites); margin-derived talus blocks (rock falls, gravity sliding of individual blocks); margin- and slope-derived clast-supported (?) megabreccias (debris flows)

Lawson, 1989; Brown and Loucks, 1993

Eberli, 1987

seismic activity (and steepening of platform margin)

Ellis et al., 1990

erosion during tectonically driven falls in relative sea level

Garcı´a-Monde´jar, 1990; Garcı´aMonde´jar and Ferna´ndez-Mendiola, 1993; Ferna´ndezMendiola et al., 1993; Rosales et al., 1994

P.A. Drzewiecki, J.A. Simo´ / Sedimentary Geology 146 (2002) 155–189

Name, age, and location

steep, grain-rich; depositional slope

Pyrenean Basin (Northern Spain; mid-Cretaceous)

steep, mud-rich; depositional/breccia bypass slope

Pyrenean Basin (Northern Spain; Late Cretaceous) Las Negras area (Southeastern Spain; Miocene)

steep; erosional slope steep, grain-rich; depositional slope

peloidal, skeletal, lithoclastic wackestone to packstone, graded skeletal packstone to rudstone (dilute turbidites); rudistfragment packstone to rudstones (gravity movement of individual clasts); pelagic mudstone to wackestone (suspension fall-out); contorted bedding (slumps); clast- to matrix-supported, margin and slope-derived breccias (debris flows) upper slope: hemipelagic sediment (suspension fall-out); bioclastic grainstone (debris flows). Lower slope: marls, hemipelagic sediment (suspension fall-out); deformed hemipelagic sediment (slides, slumps); matrix-supported, slopederived megaconglomerates (debris flows); clast-supported, matrix-rich, margin-derived megabreccias (debris flows) marls, hemipelagic sediment (suspension fall-out); deformed hemipelagic sediment (slides, slumps); clast-supported, matrixpoor, margin-derived megabreccias (rock falls, debris flows) fine-grained skeletal wackestone to packstone (hemipelagic accumulation); coarse-grained skeletal packstone to grainstone (turbidity currents ?); red-algal packstone to grainstone (in situ deposition); volcaniclastic sandstones and conglomerates (debris flows, turbidites); reef-derived clast- and matrix-supported carbonate megabreccias (debris flows); reef-derived talus (slumping of blocks at margin) upper slope: fine-grained, bioturbated periplatform ooze (suspension fall-out); hardgrounds. Lower slope: graded bioclastic sands with shallow-water allochems (turbidites); matrix-supported breccias with slope-derived clasts and muddy or grainy matrix (debris flows); periplatform ooze (suspension fall-out)

North Little Bahamas Bank (Bahamas; Modern)

steep; bypass slope

Tongue of the Ocean (Bahamas; Modern)

steep; bypass slope

upper slope: talus boulders (rockfall); fine periplatform ooze (suspension fall-out); some margin-derived sand and gravel (grain flows); hardgrounds. Lower slope: margin- and lagoon-derived skeletal and peloidal wackestone to grainstone (turbidites)

Nicaraguan Rise (Caribbean; Modern)

steep; grain-rich; depositional/erosional slope

margin- and slope-derived megabreccias (debris flows); finingupward halimeda packstone to grainstone (turbidites, debris flows?); chalk (suspension fall-out)

erosion during sea level lowstand (some rudstones derived during highstand shedding) seismic activity (possible erosion during sea level lowstand)

Carrasco-V., 1977; Enos, 1977, 1985; Enos and Stephens, 1993

seismic activity

this study

erosion during sea level lowstand

Franseen, 1989; Franseen and Mankiewizc, 1991; Franseen et al., 1993; Franseen and Goldstein, 1996 Mullins and Neumann, 1979; Mullins, 1983, 1985; Mullins et al., 1984; Harwood and Towers, 1988 Schlager and Chermak, 1979; Grammer and Ginsburg, 1992; Grammer et al., 1993

uncertain (slides are generated on slope possibly resulting from oversteepening) uncertain (oversteepening, water loading, gravitational force, differential cementation are all possible) seismic activity (along with exposure of margin and differential compaction)

this study

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Valles and Golden Lane Platforms (Eastcentral Mexico; mid-Cretaceous)

Hine et al., 1992, 1994

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160 P.A. Drzewiecki, J.A. Simo´ / Sedimentary Geology 146 (2002) 155–189 Fig. 1. Location (A), geological map (B), and aerial photograph (C) of the study area. Beds dip steeply to the south in the western portion of the map and get younger toward the south. When viewed with the north arrow down, the west side of the map is essentially a cross-section of Albian to Santonian strata. The interval of high relief ridges and valleys represents the polymictic megabreccia beds of sequence UK-2. Measured sections: SG = San Gervas, LS = La Serra (Fig. 5), LL = Llastari, MO = Monestario, SO = Sopeira (Fig. 5), and SA, SB, SC, SD, and SE are Sopeira A, Sopeira B, Sopeira C, Sopeira D, and Sopeira E, respectively. The geologic map shows the views and location of Figs. 8, 9, and 16. Location, orientation, scale, and description of the aerial photograph is provided in (A) and (B).

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Fig. 1 (continued ).

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east – west trending seaway (Pyrenean basin) between the Tethys and North Atlantic realms (Puigdefabregas and Souquet, 1986). This resulted in a dramatic change in sedimentation style and shifted the direction of sediment transport from eastward (Early Cretaceous) to northwestward (Late Cretaceous). The Sopeira region is located at the southern edge of the fault-bounded basin, and was affected by the faulting during the Albian and Cenomanian. In the Early Cenomanian, deposition was restricted to the basin, while exposure and tilting of the footwall block to the south resulted in erosion of Lower Cretaceous and Upper Jurassic strata. During the Middle Cenomanian, the footwall block was flooded, forming a major angular unconformity between Cenomanian strata and the underlying Jurassic through Lower Cretaceous footwall strata. Shallow Upper Cretaceous carbonates are restricted to the southeast (footwall block), while a deep basin persisted to the northwest (Simo´, 1986, 1989).

The Sopeira region (Fig. 1) experienced three phases of slope development. The first phase occurred during wrench tectonics, and is characterized by evolution of the platform from ramp-like in the Early and Middle Cenomanian to a steep, rimmed shelf in the Late Cenomanian to Early Coniacian through both depositional and erosional processes. The second phase is characterized by partial burial of the steep margin by pelagic sediments, and a return to a more gentle ramp-like profile during the Late Coniacian and Early Santonian. The third phase (Late Santonian) is characterized by large-scale erosion of Cenomanian to Lower Santonian carbonates and the emplacement of siliciclastic turbidites associated with the initiation of Late Cretaceous foreland basin development. Thus, the slope was a long-lived (Cenomanian to Santonian) region of tectonically induced instability, erosion, and redeposition. This paper examines the depositional processes, lithologies, and margin morphologies that characterize each phase of evolution.

Fig. 2. Reconstructed stratigraphic cross-section for Cenomanian to Santonian strata (after Simo´, 1993). Numbers 1 to 6 refer to sequences UK-1 to UK-6, respectively. Note that the shallow carbonate platforms backstep through time, in response to flexural subsidence induced by foreland basin development. The study area is in the northwestern portion (Sopeira region) of this diagram. See Simo´ (1993) for a regional synthesis.

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3. Previous work and sequence stratigraphic setting Facies descriptions, sequence stratigraphy, biostratigraphy, lithostratigraphy, and chemostratigraphy of Upper Cretaceous strata in the study area are provided by Misch (1934), Rosell (1967, 1970), Souquet (1967), Mey et al. (1968), Simo´ (1986, 1989, 1993), Caus and Go´mez-Garrido (1989), Soriano (1992),

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Caus et al. (1993, 1997), Booler (1994), Rodrigues de Miranda (1994), Drzewiecki (1996), and Drzewiecki and Simo´ (2000). The slope deposits described in this paper belong to six depositional sequences (Figs. 1, 2, 3, and 4). From oldest to youngest, they are: UK-1 (Lower to Upper Cenomanian), UK-2 (Upper Cenomanian), UK-3 (Uppermost Cenomanian to Middle Turonian), UK-4 (Upper Turonian to Middle Coniacian), UK-5 (Upper Coniacian to Middle

Fig. 3. Chronostratigraphic diagram for Albian through Santonian strata in the field area. Measured sections: SG = San Gervas, LS = La Serra (Fig. 5), LL = Llastari, MO = Monestario, and SOP = Sopeira (Fig. 5). LST = lowstand systems tract, TST = transgressive systems tract, and HST = highstand systems tract. Absolute dates and planktonic foraminifera biozones are from Harland et al. (1990). Location of sections shown in Fig. 1.

164 P.A. Drzewiecki, J.A. Simo´ / Sedimentary Geology 146 (2002) 155–189 Fig. 4. Restored cross-section for slope and margin strata preserved in the field area showing the distribution of lithofacies and observed stratal relationships. Note that the matrix-rich, polymictic megabreccias are concentrated in certain stratigraphic horizons in sequence UK-2. Locations of sections are shown in Fig. 1. Measured sections: SG = San Gervas, LS = La Serra (Fig. 5), LL = Llastari, MO = Monestario, and SO = Sopeira (Fig. 5). mfi = maximum flooding interval.

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Santonian), and UK-6 (Upper Santonian to Campanian). The sequences have been defined by Simo´ (1986, 1989, 1993), Soriano (1992), Drzewiecki (1996), and Drzewiecki and Simo´ (2000) based on the occurrence of exposure surfaces, depositional hiatuses, facies transitions, stratal geometries, chemostratigraphic horizons, and submarine erosion surfaces (Figs. 1, 2, 3 and 4). Figs. 2 and 3 display the general sequence stratigraphic and chronostratigraphic framework of the study area and associated shallow-water deposits. Sequence UK-1 contains a 160-m-thick basinally restricted wedge of marl, a 30 m transgressive flooding interval, and a 110-m-thick package of prograding highstand carbonates composed of lagoonal, backmargin, margin, slope, and basin facies. The margin facies, exposed on the eastern edge of the study area, is composed of coarse bioclastic packstone to grainstone sand shoals and small, discontinuous caprinid rudist biostromes. The boundary between sequences UK-1 and UK-2 is represented by a submarine erosion surface and the backstepping of facies on the slope (Figs. 3 and 4), and by a subaerial exposure surface on the shelf (Soriano, 1992; Drzewiecki and Simo´ , 2000). UK-2 contains a 20-m-thick lowstand systems tract composed of carbonate megabreccias and a 200 m prograding highstand systems tract that contains the same facies belts as sequence UK-1. Transgressive deposits have not been recognized in the slope succession because of extensive redeposition. UK-2 highstand deposits are thin on the shelf. Shelf-margin and slope deposits are thicker in comparison, and were deposited basinward of the UK-1 margin deposits (Figs. 2 and 3). Drowning of sequence UK-2 in the latest Cenomanian resulted in a major backstepping of carbonate facies, and a shift from benthic deposition to pelagic deposition (Figs. 2, 3 and 4). Across the shelf, slope, and basin, this drowning unconformity is overlain by calcisphere-rich wackestone to packstone (sequence UK-3) deposited in response to increased trophic resources (Drzewiecki and Simo´, 1997). These deposits thin from 60 m on the inner shelf to less than 12 m over the margin of sequence UK-2, and thicken back to 80 m on the slope. The boundary between sequences UK-3 and UK-4 is a paleosol on the inner shelf (Soriano, 1992; Drzewiecki and Simo´, 2000) and a flooding surface on

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the outer shelf and basin as a result of rapid flexural subsidence. Continuous backstepping of shallow carbonates occurred through the Turonian to Campanian, in part due to increased rates of flexural subsidence and basinward tilting. In the mid-Turonian, shallow-water carbonate deposition shifted to a position about 10 km southeast of the field area (Fig. 2). Deposits of sequence UK-4 (Simo´, 1989, 1993; Booler, 1994) are characterized by a thick shallow-water platform in a landward position, condensed deposition over the drowned shelf of sequence UK-3, and a thin wedge of calcareous shales, marls, and dark gray pelagic wackestones in the basin (Figs. 1, 2, 3 and 4). A subaerial exposure surface developed on the shelf of UK-4 (Booler, 1994), while in the basin, a marine erosion surface occurs above sequence UK-4, truncating portions of sequences UK-1 to UK-4. This surface is overlain by a basinally restricted (lowstand) wedge of interbedded megabreccias and pelagic limestones of sequence UK-5 (Figs. 1, 2, 3 and 4; Simo´, 1989, 1993). A thick package of transgressive and highstand pelagic carbonates was deposited above the wedge and is correlative to a cyclic, shallow-water carbonate platform exposed about 10 km southeast (landward) of the margin of sequence UK-4 (Fig. 2). Finally, an erosional surface with more than 800 m of relief cuts through Lower Santonian to Cenomanian strata (Figs. 1, 2, 3 and 4), and is filled with Santonian siliciclastic turbidites that are time-equivalent to a thick shallow-water carbonate ramp system that is exposed about 15 km south (landward) of the UK-5 margin (Fig. 2).

4. Slope lithologies and depositional interpretations Slope depositional environments commonly contain a wide range of reworked sediment types, and numerous authors have attempted to classify these sediment gravity-flow deposits based on flow rheology, transport process, or support mechanism (e.g. Dott, 1963; Middleton and Hampton, 1973, 1976; Carter, 1975; Nardin et al., 1979; Lowe, 1979; Hansen, 1984; Postma, 1986; Peirson and Costa, 1987). These schemes are difficult to apply to ancient rocks, since they are based on attributes that are not directly preserved in the rock record. This paper utilizes a

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simple classification scheme which considers the depositional product along with the process (Table 2; Rupke, 1978; Stow, 1986; Nemec, 1990; Martinsen, 1994). Table 3 summarizes the eight major slope lithologies in the Cenomanian through Santonian sequences. The lithologies represent both autochthonous (primary) and allochthonous (redeposited) depositional products. Allochthonous lithologies are defined on the basis of their internal fabric, depositional geometries, and inferred transport mechanism. Figs. 3, 4, and 5 display the vertical and lateral distribution of these lithologies. 4.1. Autochthonous deposits The marls/carbonate mudstone facies (Fig. 6) consist of tan to dark gray, thin bedded marl, chalk, calcareous shale, and carbonate mudstone. Glauconite is a common constituent in some intervals. Pelagic foraminifera and calcispheres are the dominant biotic constituents, but nautiloids and whole echinoderm tests are locally abundant. This facies occurs in laterally continuous thin to medium beds, which are commonly arranged into marl –chalk couplets. These sediments are interpreted to represent deposition from suspension under quiet, deep-water conditions. The fine hemipelagic wackestone facies is tan to light gray, and is composed of carbonate mudstone to wackestone containing pelagic foraminifera, calci-

spheres, and minor mollusks and echinoderms. The sediments are typically bedded on the decimeter to meter scale, and are bioturbated within beds. Hemipelagic wackestones are interpreted to have been deposited from a combination of suspension fall-out and in situ benthic deposition in a basin margin environment. The Pithonella packstone facies (Fig. 7) is composed of dark gray wackestone to packstone containing abundant calcispheres of the genus Pithonella and some pelagic foraminifera. Locally, chert nodules and glauconite are present. Bedding is on the decimeter to meter scale, and beds are typically bioturbated. A 1.5m-thick interval of laminated, organic-rich sediment is preserved in the slope succession at the Cenomanian – Turonian boundary. The Pithonella packstone is interpreted to have been deposited from suspension under nutrient-rich, high productivity conditions (Drzewiecki and Simo´, 1997). 4.2. Allochthonous deposits The bioclastic grainstone to rudstone facies is characterized by gray peloidal – skeletal packstone, grainstone, and rudstone, primarily composed of mollusks (including rudists), echinoderms, corals, benthic foraminifera, pelagic foraminifera, and calcispheres. Grain size ranges from fine sand to about 5 cm, and beds are typically 0.1 –3 m thick. This facies is the result of combined autochthonous and allochthonous processes. The pelagic component is interpreted to be

Table 2 Definitions of sediment gravity-flow processes used in this paper Rock fall — individual pieces of lithified rock are dislodged and transported downslope (Martinsen, 1994). No support mechanism (minor contributions from grain collisions). Rock fall deposits form a clast-supported, matrix-lean talus cone at the base of the slope. Debris fall (avalanche) — downslope movement of dispersed debris, with freely moving grains responding to the downslope pull of gravity and a minor contribution from grain collisions (Nemec, 1990; Martinsen, 1994). Debris flow deposits form coarse, clast-supported bodies at the base of slopes. Inverse grading is common, and coarser clasts may travel further into the basin. Slide — downslope displacement of sediment along a distinct shear surface with little or no internal deformation of transported sediment (Martinsen, 1994). Support mechanism is cohesion. Slide deposits form a single transported sheet with original bedding preserved. Slump — downslope movement of sediment along a distinct shear surface with significant internal deformation of transported sediment (Martinsen, 1994). Support mechanism is cohesion. Slump deposits form lenticular units of deformed sediment with contorted and folded remnants of bedding. Debris flow — cohesive mass of unsorted debris, in a cohesive matrix (Martinsen, 1994). Support mechanism is cohesion and buoyancy provided by matrix. Debris flow deposits form lenticular or sheet-like units of coarse debris and mud that may be clast- or matrixsupported. Internal fabric may be chaotic or graded. Turbidity current — transport of generally low-concentration sediment suspensions supported by fluid turbulence (Martinsen, 1994). Turbidites are sharp-based, graded beds that may exhibit partial or full Bouma sequences.

Table 3 Slope lithofacies of the sequences UK-1 through UK-5 Description

Geometry

Internal Fabric

Depositional Environment

Sequences

Depositional Processes

Autochthonous lithofacies Marl-carbonate mudstone

tan to dark gray mudstone to sparse wackestone

tabular beds

upper to lower slope

UK-1, UK-4, UK-5

suspension fallout

fine-grained, bioclastic wackestone composed of calcispheres and pelagic foraminifera gray packstone composed of Pithonella calcispheres and some pelagic foraminifera

tabular beds

thinly bedded to massive; bioturbated bedded; massive and bioturbated within beds

most common in middle slope (upper to lower slope) upper, middle, and lower slope

UK-1, UK-2, UK-4, UK-5

suspension fallout

UK-3

suspension fallout

Hemipelagic wackestone

Pithonella packstone

Allochthonous (redeposited) lithofacies Bioclastic grainstone fine to coarse bioclastic packstone to grainstone composed of peloids, mollusks, echinoderms, and pelagic foraminifera Deformed hemisame as fine hemipelagic pelagic wackestone wackestone, but with deformed and contorted bedding Slope-derived pelagic wackestone matrix megaconglomerate with large (up to 6 m) hemipelagic clasts Matrix-rich, polymictic megabreccia

Matrix-poor, polymictic megabreccia

pelagic wackestone matrix with large (up to 5 m) hemipelagic and margin grainstone clasts large (up to 5 m) shallow water grainstone clasts in a sparse hemipelagic wackestone matrix

tabular beds

bedded; massive and bioturbated within beds

tabular beds

bedded; massive and bioturbated within beds

upper to middle slope

UK-1, UK-2

storm-initiated fine-grained debris flows

individual slump masses, up to 20 m thick and 100 m wide sheet-like bodies (up to 30 m thick and 600 m wide)

contorted bedding; bases may be erosive

lower and middle (?) slope

UK-1, UK-2, UK-3, UK-5

submarine slumps and slides

massive; chaotic; non-erosive bases; matrix-supported

lower slope

UK-1, UK-2

massive; chaotic; erosive bases; matrix- to clastsupported massive, graded, or inversely graded; erosional bases; clastsupported

lower slope

UK-2, UK-4

submarine debris flows originating on slope submarine debris flows

upper to middle slope

UK-5

channelized and sheet-like bodies (up to 30 m thick and 300 m wide) talus piles and sheet-like bodies (up to 50 m thick and 600 m wide)

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Name

submarine rock falls, debris falls, and possibly debris flows 167

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Fig. 5. Stratigraphic columns for the Sopeira and La Serra sections, representing lower and upper slope environments, respectively. Locations are shown in Fig. 1.

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Fig. 6. Autochthonous lithologies. (A) Outcrop photograph of Sopeira marls that form the basinal deposits of sequence UK-1. Alternating recessive-resistant packages are marl – chalk couplets. The relative proportions of marl and chalk vary, forming thirteen fining-upward packages. Youngest strata to the left. Arrow points to a scale that is 2 m in length. (B) Hemipelagic sediments from sequence UK-5. Note that intraformational erosion surfaces (between sets of arrows) indicating slope reworking. Beds are on the order of 10 – 30 cm thick.

the result of suspension deposition and the benthic component was derived either in situ or transported from the shelf margin to the slope through storminitiated, fine-grained cohesive and cohesionless debris flows. Pervasive bioturbation within beds has obscured depositional textures. The deformed hemipelagic wackestone facies (Figs. 6, 7, 8 and 9) represents the portion of hemi-

pelagic wackestone, marl-carbonate mudstone, and Pithonella packstone facies that have been dislocated from their original depositional position and redeposited by sediment gravity flow processes. Lithologically, they are identical to their parent lithofacies, but internally, original bedding is slightly deformed to highly contorted. These deposits are typically preserved as lens-shaped bodies up to 20 m thick and

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Fig. 7. Outcrop photograph of Pithonella packstone lithology of sequence UK-3. Note the intraformational erosion surface (between arrows) the separates older slumped limestone (at right) from younger bedded limestone (at left). This facies was deposited from suspension. This photograph shows about 5 m of the section.

100 m wide (Fig. 8). Slide dislocation surfaces and intraformational erosion surfaces are common on the slope (Figs. 6, 7 and 8) and truncation surfaces are present within individual bodies, but there is remark-

ably little shear deformation along the dislocation surfaces. This lithology is interpreted to represent redeposition of slope material by submarine slides and slumps.

Fig. 8. Slumped beds in the lower slope region of sequence UK-2 (see Fig. 1 for location). Note that the slumped beds (a) have a different orientation than the overlying undeformed beds (b). Matrix-rich polymictic megabreccia beds (b and c) weather more resistant, while the covered intervals represent hemipelagic sediments, deformed hemipelagic sediments, and slope-derived megaconglomerates. A large contorted clast from one of the megaconglomerates is visible on the left portion of the photograph (d). This photograph is approximately 340 – 420 m of the Sopeira section (Fig. 5). Older strata are to the left.

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Fig. 9. Photograph (A) and interpretation (B) of the distribution of lower slope lithologies in sequence UK-1. View (looking south) is of the prograding highstand systems tract of sequence UK-1 and the lowstand systems tract of sequence UK-2 (thick line indicated by arrows is the sequence boundary). White bedded strata at the base of the photograph are marls of the underlying lowstand and transgressive systems tracts of UK-1. Light gray represents hemipelagic sediments, deformed hemipelagic sediments (slumps and slides), and slope-derived megaconglomerates. Dark gray is matrix-rich polymictic megabreccia with clasts containing evidence of subaerial exposure. Locations (a) and (b) show submarine slide and slump deposits. Fig. 10 is taken at location (b). Note that the prograding highstand wedge downlaps the transgressive systems tract to the west, the bed labeled (c) pinches out to the right. See Fig. 1 for location of view. Strata in the center of the photograph represent 160 – 315 m of the Sopeira section (Fig. 5).

The slope-derived megaconglomerate facies (Fig. 10) has a composition similar to hemipelagic wackestone, and marls-carbonate mudstone for both matrix and clasts. Clasts are rounded to subangular, and range in size from 1 cm to 7 m. They are commonly tabular in shape and represent reworked and contorted

fragments of slope beds. Contorted clasts indicate that they were only semilithified before erosion and transport. The matrix is structureless or shows contorted bedding. The conglomerates have a chaotic internal fabric and lack any evidence of grading. They are typically matrix-supported, but can be clast-supported

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Fig. 10. Photograph (A) and interpretation (B) of slope-derived megaconglomerates from the lower slope of sequence UK-1 (see Fig. 9 for location). The megaconglomerate interval is approximately 2 m thick and is bounded by undeformed marls and hemipelagic sediments. The clasts in the megaconglomerate are tabular and represent fragments of slope bedding. The matrix is structureless.

in rare instances. They form sheet-like beds up to 1 km wide and 30 m thick that are characterized by nonerosional bases and flat tops. The deposits are interpreted to have been transported as debris flows that resulted from slumping and redeposition of slope sediments. A subgroup of this lithology contains sparse small (1 –5 cm), rounded mudstone clasts and occurs in discontinuous beds (0.5 – 2 m thick), commonly located immediately above matrix-rich, polymictic megabreccias (see below).

The matrix-rich, polymictic megabreccia facies (Fig. 11) contains clasts that range from 1 cm to 5 m in diameter and are derived from a variety of slope and margin lithologies. Clasts represent the marlcarbonate mudstone, hemipelagic wackestone, and bioclastic grainstone lithofacies, but the most common clast lithologies are tan to brown, coarse, well-sorted grainstones, and caprinid rudist rudstones. The last two lithologies have not been observed in situ in the field area, and are interpreted to characterize the

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Fig. 11. Photograph of matrix-rich polymictic megabreccias from the lower slope of sequence UK-2. Texture is clast-supported, and some of the larger clasts are outlined. Clasts can be up to 5 m in diameter, and are typically composed of tan bioclastic grainstone from the margin. They are lighter colored than the matrix and have a blocky fracture. Note the person for scale.

platform margin. Slope-derived clasts are subangular to rounded and appear to have been semilithified before transport. Margin-derived clasts are angular to subangular, and contain evidence of early marine (and occasionally meteoric) cements, that indicate the parent sediment of the clasts was lithified prior to erosion and transport. The internal fabric of this lithofacies is chaotic, and the matrix is a mudstone to sparse wackestone. In sequence UK-2, the matrix contains benthic foraminifera indicative of shallow, semi-restricted marine environments (miliolids and praealveolinids), suggesting that the shelf was flooded during megabreccia deposition. The megabreccias have either channelized (up to 300 m wide) or sheet-like (up to 1 km wide) geometries that attain thicknesses up to 25 m. Both channelized and sheetlike deposits have erosional bases. The breccias are interpreted to represent debris flow deposits that originated by collapse of the platform margin, bypass of the upper slope, and deposition on the lower slope. Although they are generally clast-supported now, the abundance of matrix suggests that this lithology was matrix-supported during deposition. Post-depositional dewatering and compaction probably resulted in the present clast-supported texture. Furthermore, clast

edges are commonly stylolitized, enhancing the apparent clast-supported texture. Some of the clasts in the lowest matrix-rich, polymictic megabreccia bed of sequence UK-2 contain evidence of dissolution porosity filled by gray nonfossiliferous internal sediment (carbonate mudstone) and a bladed cement that is possibly meteoric. These features are interpreted to be a result of post-depositional exposure and erosion of the UK-1 margin. The matrix-poor, polymictic megabreccia facies (Figs. 12 and 13) is distinguished from other clastsupported megabreccias by the relative lack of matrix. Clasts are subangular to angular, and are almost exclusively composed of tan to brown, coarse, wellsorted grainstone and caprinid rudist rudstone (from the margin of sequences UK-1 and UK-2), and Pithonella packstones (from sequence UK-3). Matrix is rare, but where observed, is mudstone. These megabreccias occur in two distinct modes: (1) sheetlike beds (up to 600 m long and 10 m thick) of graded or inversely graded megabreccias, and (2) a thick chaotic talus pile. The talus pile is located on the upper slope, whereas the bedded megabreccias occur on the lower and middle slope. Bedded megabreccias thin basinward until they are represented by just a few

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Fig. 12. Photograph (A) and interpretation (B) of a matrix-poor polymictic megabreccia bed from the lowstand systems tract of sequence UK-5. The bed is oriented nearly vertically, and the view is looking at the top of a bedding surface from above. This photograph shows the erosional remnants of a graded turbidite cap above the breccia bed (massive weathering texture) in the foreground. Behind that is the upper bedding surface of the megabreccia showing numerous clasts that range up to 50 cm in size. Width of this photograph is about 20 m.

outlier boulders. These beds are commonly capped by a thin (less than 20 cm) layer of graded intraclastic packstone. Matrix-poor, clast-supported megabreccias are interpreted to have been deposited by rock falls and debris falls derived from the margins of sequences UK-1, UK-2, and UK-3, which were exposed on the sea floor. The matrix-poor, polymictic megabreccias are characterized by abundant clasts with large (up to 10

cm) dissolution vugs that have been subsequently filled with internal sediment and possible meteoric cements (Fig. 13). The internal sediment is gray, nonfossiliferous carbonate mudstone and tan Pithonella packstone and wackestone. The vugs are interpreted to have formed during subaerial exposure of the shelf and margin of sequence UK-1 (only margin deposits from UK-1 show evidence of subaerial exposure), that were later filled during the transgres-

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sions of sequences UK-2 and UK-3. With the exception of lowstand deposits from sequence UK-2, they are the only breccias with clasts that show evidence of subaerial exposure.

5. Distribution of lithologies and slope evolution The lateral and temporal distribution of slope lithofacies (Figs. 3, 4, 5 and 9) are intimately related to the evolution of the slope, which in turn responded to tectonic, eustatic, and environmental changes. The following discussion incorporates aspects of slope evolution and lithofacies distribution by addressing them within a sequence stratigraphic framework (Drzewiecki and Simo´, 2000). An interpretation of depositional processes for the various lithofacies is included. A depositional model, based on facies distribution patterns, depositional processes, and tectonic evolution is provided for the slope region of each sequence (Fig. 14). Carbonate slope models have been extensively reviewed (McIlreath and James, 1978, 1984; Cook and Mullins, 1983; Enos and Moore, 1983; James and Mountjoy, 1983; Hine and Mullins, 1983; Mullins and Cook, 1986; Tucker and Wright, 1990; Coniglio and Dix, 1992). Slopes typically display a complex mosaic of autochthonous and allochthonous deposits that represent a wide range of depositional processes, making it difficult to place individual slopes into the generalized schemes. McIlreath and James (1978, 1984), Crevello and Schlager (1980), James and Mountjoy (1983), Enos and Moore (1983), and Coniglio and Dix (1992) discuss two end member models for carbonate slopes: (1) a depositional slope that is characterized by allochthonous sediments attached to the margin that decrease in grain/clast size downslope and downlap basin strata, and (2) a bypass slope where allochthonous sediment bypasses the upper slope and onlaps the lower slope. Depositional slopes are dominated by margin-derived sediment that may be reworked more than once, and bypass slopes contain both margin- and slope-derived reworked sediment. Depositional slopes typically have low gradients and bypass slopes typically have either steep slope angles or an erosional scarp. Mullins and Cook (1986) suggest that bypass slopes may follow either a submarine fan model when sediment is derived at a

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point source, or a carbonate apron model when sediment is derived along a line source. For the purposes of this study, three slope types are recognized (Fig. 15): (1) depositional slope, (2) onlapping slope, and (3) bypass slope. Depositional slopes are characterized by sedimentation across the entire slope with only minor reworking. Allochthonous sediments derived from the margin are minor. Hydrodynamic sorting of sediment results in a downslope decrease in grain size. Coarse debris is retained at the margin and upper slope, and mud is carried in suspension to the lower slope and basin. Onlapping slopes are characterized by deposition of marginderived sediment and pelagic carbonate mud across the entire slope, interbedded with margin-derived megabreccias that onlap the middle and lower slope. Megabreccia was deposited on the middle and lower slope from debris flows that eroded the margin and bypassed the upper slope. Bypass slopes are characterized by an absence of any sedimentation on the upper slope. They occur on both oversteepened upper slopes and along erosional scarps. 5.1. Sequence UK-1 The lower portion of sequence UK-1 is composed of 165 m of marl-carbonate mudstone facies (Fig. 6), which forms continuous, flat-lying beds that are interpreted to onlap underlying strata (Fig. 4). These sediments represent a basinally restricted wedge that was deposited during a relative sea level lowstand. A 35-mthick transgressive systems tract (TST) overlies these strata, and contains dark gray calcareous shales with glauconite and cephalopod shells (marl/carbonate mudstone lithofacies). The highstand systems tract (HST) is composed of a prograding succession (65 m thick at Sopeira) of the marls-carbonate mudstones, hemipelagic wackestones, deformed hemipelagic wackestones, and slope-derived megaconglomerates that downlap the transgressive systems tract with a measured angle of 12° (Figs. 4, 6, 8, 9 and 10). Deposition from suspension was the dominant process operating during all systems tracts of sequence UK-1. However, during the highstand, most prograding slope deposits were modified by submarine slumps and slides. Deformed hemipelagic sediments are interpreted to have originated as slides and slumps in the middle and lower slope region, where the slope (mini-

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Fig. 14. Interpreted evolution of the carbonate slope and margin environments from the Cenomanian through the Coniacian. Note the influence of normal faulting on slope morphology, facies distributions, depositional processes, and stratal relationships. Sequences UK-1 to UK-5 represent phase 1 (rifting) of slope evolution. Sequence UK-5 represents phase 2 (burial), and sequence UK-6 is phase 3 (exhumation).

mum of 12°) was too steep for the stability of mud-rich deposits. On the lower slope, deformed hemipelagic sediments occasionally grade laterally downslope into

slope-derived megaconglomerates. Clasts in the megaconglomerates represent redeposited and contorted fragments of bedding, and have the same composition

Fig. 13. Matrix-poor, polymictic megabreccias. (A) Outcrop photograph showing the texture of the talus pile megabreccias from the lowstand systems tract of sequence UK-5. Note the clast-supported nature and the lack of matrix. These breccias are interpreted to have been deposited by rock falls and debris falls. Ax is about 50 cm in length. (B) Polished slab of a clast from the matrix-poor, polymictic megabreccia of sequence UK-5 showing a large dissolution vug (a) in the grainstone host rock (b) derived from the margin of sequence UK-1. The dissolution vug is lined with bladed cements and filled with laminated mudstones. Note that the laminated mudstone shows rotation and microfaulting, probably resulting from rotation of the clast when the mudstone was partially lithified. The vug was formed during a subaerial exposure event following deposition of sequence UK-1.

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Sequence UK-1 is interpreted to be deposited on a depositional slope that had a gentle gradient during deposition of the basinally restricted wedge and TST, and a steeper gradient (about 12°) during the HST. The distribution of redeposited sediments suggests that slope stability was maintained until the late HST. At this point, the slope gradient increased to a point of instability resulting in the redeposition of slope sediments (Figs. 9 and 14). 5.2. Sequence UK-2

Fig. 15. Schematic diagram of the types of slopes observed in the study region. Sequences UK-1 and UK-3 are represented by the depositional slope model. Sequence UK-2 is an onlapping slope, where coarse megabreccias appear to onlap the finer-grained lower slope lithologies. Sequences UK-4 and UK-5 are bypass slopes. Sediment bypasses a steep depositional slope in sequence UK-4, and coarse debris abuts an erosional scarp in sequence UK-5.

as the equivalent upslope slump or slide masses. This suggests that the megaconglomerates originated as submarine slumps and slides, but experienced a greater degree of internal deformation during transport. A similar relationship between debris flow deposits and submarine slides and slumps was observed by Cook and Taylor (1977) in Cambrian to Ordovician slope strata in Nevada, USA. Upslope, hemipelagic wackestone and bioclastic grainstone facies dominate, and depositional processes include a combination of in situ benthic deposition, deposition from suspension, and mass gravity flows (including localized fine-grained cohesive and cohesionless debris flows) that transported carbonate sand from the margin to the upper slope. This coarse material is stable at higher depositional angles than is the fine material on the lower slope (Kenter, 1990), and was not extensively reworked.

A matrix-rich, polymictic megabreccia bed immediately overlies the lower sequence boundary of UK2. Margin-derived clasts in this bed have dissolution porosity and meteoric cements, indicating that they originated from subaerial exposure and erosion of the margin of sequence UK-1. The bed is overlain by amalgamated hemipelagic wackestone, deformed hemipelagic wackestone, slope-derived megaconglomerates and matrix-rich, polymictic megabreccias (Fig. 9). The megabreccias of this amalgamated package have erosional bases that cut down into the sequence boundary, and are similar in lithology to the basal megabreccia bed, but lack evidence of subaerial exposure. The entire unit is 30 m thick, erodes into and onlaps the lower slope of sequence UK-1, and is interpreted to be the LST of sequence UK-2 (Fig. 4). In the lower slope region, no TST was recognized as a result of extensive redeposition of sediments. The LST is overlain by about 200 m of mixed lithofacies (including hemipelagic wackestone, reworked hemipelagic wackestone, slope-derived megaconglomerates, and matrix-rich, polymictic megabreccias; Figs. 4, 8 and 11) that represent a prograding HST. These strata are deposited at an angle of about 12° with respect to the TST of sequence UK-1. Margin-derived breccia clasts lack evidence of subaerial exposure. The megabreccia deposits are typically channelized, and tend to be concentrated at specific stratigraphic intervals (Figs. 1, 3 and 4). The breccias onlap the slope, and through time, younger breccias are deposited progressively closer to the platform margin (Figs. 1, 3 and 4). Mixed hemipelagic wackestone, deformed hemipelagic wackestone, and slope-derived megaconglomerates occur between the polymictic megabreccia beds

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(Fig. 9). Upslope, redeposited sediments become less common, and hemipelagic wackestone and bioclastic grainstone lithofacies dominate. Lower slope depositional processes that operated during sequence UK-2 are similar to those during UK1. Background sedimentation from suspension is present in all systems tracts, and reworking of the sediments occurred as submarine slumps, slides, and debris flows, forming the reworked hemipelagic wackestones and slope-derived megaconglomerates. Deposition of matrix-rich, polymictic megabreccia interrupted the background processes. Megabreccias were eroded from the platform margin and upper slope, bypassed the upper slope, and were deposited on the lower slope by debris flows. The matrix and many of the clasts are composed of carbonate mudstone and hemipelagic wackestone, suggesting that slope lithologies were eroded and incorporated into the debris flows as they were transported downslope. The debris flow deposits are commonly confined to channels (Figs. 4 and 9). However, they also occur in more sheet-like geometries, particularly in the lower slope. Breccias are concentrated in several stratigraphic horizons (Figs. 1, 3 and 4), suggesting that erosion of the margin was more common at certain times than at others. A close lateral association between some of these megabreccias and deformed hemipelagic sediments or slope-derived megaconglomerates suggests that the margin-derived debris flows may have triggered submarine slumps, slides, and debris flows on the slope. Observations from the middle slope were limited by vegetation, but the upper slope of sequence UK-2 experienced deposition from suspension, in situ benthic deposition, and grain-rich sediment gravity flows (fine-grained cohesive and cohesionless debris flows). Vertical trends in the composition of the matrixrich, polymictic megabreccia clasts suggest that erosion was concentrated on the slope in the early part of the UK-2 sequence highstand, and progressively migrated upslope, affecting shallower environments through time. The basal breccia bed contains clasts derived from the upper slope and margin of sequence UK-1 that contain dissolution porosity and possible meteoric cements. This unit is interpreted to be a lowstand deposit of sequence UK-2. Megabreccias immediately overlying the lowstand deposits contain only upper slope lithologies. Younger beds contain

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margin-derived clasts that become progressively more abundant through time (up to 90% of clasts). Finally, the uppermost megabreccia bed of sequence UK-2 contains some clasts with back-margin lithologies. This implies that erosion of the lower slope resulted in progressive instability and back-cutting of the slope and shelf. A steep slope persisted throughout deposition of sequence UK-2, and is interpreted to be a result of a normal fault located beneath the platform margin (Fig. 1). Redeposited sediments constitute a major portion of the slope lithologies, particularly on the lower slope. Background slumping and sliding on the slope was periodically interrupted by debris flows that resulted from collapse of the margin, bypass of the upper slope, and deposition on the lower slope (Fig. 14). Sequence UK-2 is interpreted to represent an onlapping slope, and through time, clast-supported, polymictic megabreccias were deposited progressively closer to the margin (Figs. 1, 3 and 4). Apparent backstepping of margin-derived megabreccias may be attributed to: (1) erosional back-cutting of the margin and associated backstepping of slope facies, (2) a decrease in slope angle through time that reduced the gravitational driving force and allowed debris flows to ‘‘freeze’’ progressively earlier and earlier, or (3) a progressive decrease in the matrix abundance and subsequent loss of buoyancy support. 5.3. Sequence UK-3 Sedimentation style changed dramatically between sequences UK-2 and UK-3 as a result of eutrophication of the water column, increased organic productivity, and deposition of the Pithonella packstone lithofacies across the entire slope (Drzewiecki and Simo´, 1997). The slope deposits of UK-3 form a wedge of sediment that thins from about 80 m at the Sopeira section to about 20 m at the La Serra section (Fig. 5). Depositional thinning was enhanced by erosion in the upper slope environment. Transgressive and highstand systems tracts for this sequence are composed of the Pithonella packstone and reworked hemipelagic wackestone facies. Stratigraphic relationships constrained by carbon isotope stratigraphy confirm the presence of onlapping geometries defining the TST (Drzewiecki and Simo´, 2000). Progradational geometries characterize the HST on the shelf, but are

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difficult to recognize on the slope. No LST has been recognized. Deposition from suspension occurred in both the TST and HST of sequence UK-3, forming a pelagic drape over antecedent topography. Deformed Pithonella packstone beds (Fig. 7) are common, and are interpreted to have originated as submarine slides and slumps in the upper, middle, and lower slope regions. Slide and slump scars and intraformational erosion surfaces are common in both the lower and upper slope in well-exposed portions of this sequence (Fig. 7). During sequence UK-3, a pelagic drape of calcisphere-rich packstones was deposited across the shelf and slope by continual settling from suspension. Depositional thinning of sequence UK-3 over the margin of sequences UK-1 and UK-2 indicates that the margins continued to have high topographic relief on the sea floor (Fig. 14). Submarine slumping and sliding in sequence UK-3 suggest a steep slope angle was maintained. UK-3 is interpreted to represent a depositional slope that experienced some sediment redeposition. There is no evidence of progradation on the slope, and slope breccias are absent. 5.4. Sequence UK-4 Sequence UK-4 is composed of the marl-carbonate mudstone lithofacies, with thin (1– 2 m thick), discontinuous channels (5 – 10 m wide) filled with matrix-rich, polymictic megabreccias (Figs. 4 and 5). The breccias differ from older polymictic breccias in that they contain clasts derived from the Pithonella packstone facies (UK-3) and from lithologies in UK-1 and UK-2. These deposits form an onlapping wedge of sediment about 30 m thick at Sopeira that rapidly pinches out upslope, suggesting that a relatively steep depositional gradient was still maintained. It is not possible to determine whether the strata represent lowstand, transgressive, or highstand systems tracts, but they are restricted to the basin in the same manner as the LST deposits of sequences UK-1 and UK-2. Deposition from suspension was responsible for the marl-carbonate mudstone lithofacies. This background sedimentation was punctuated by deposition of matrix-rich, polymictic megabreccias from submarine debris flows. The breccias were derived by erosion of the platform margin and upper slope, bypass of the upper slope, and deposition on the lower slope.

Erosion during transport led to the incorporation of carbonate mudstone clasts into this lithology. The abundance of glauconite in some clasts suggests erosion and reworking of the drowned shelf (sequence UK-3), where a similar lithology occurs, suggesting that the back-cutting trend observed in sequence UK-2 continued at this time. Slope deposits of sequence UK-4 are limited to the lower slope region, indicating a steep slope angle and bypass of the upper slope. Movement along the fault beneath the margin of sequences UK-1 and UK2 is interpreted to have increased the depositional relief in the upper slope region (Fig. 14). UK-4 is interpreted to represent an oversteepened bypass slope. 5.5. Sequence UK-5 The lower portion of sequence UK-5 is a 200-mthick package of interbedded marl-carbonate mudstones and matrix-poor, polymictic megabreccias (Fig. 5) that progressively onlaps sequence UK-4, sequence UK-3, and the exposed margins of sequences UK-2 and UK-1 (Fig. 4). The package is restricted to the basin, and is interpreted to represent the LST of sequence UK-5. In the basin, the megabreccias form distinct graded, inversely graded, or chaotic beds (5 – 15 m thick) that directly onlap the underlying sequences (Figs. 1 and 12). Landward, the breccias form a 60m-thick unit that directly abuts the eroded margins of sequences UK-1 and UK-2 (Figs. 3, 4 and 13). In texture, the deposits resemble a talus pile that forms at the foot of a cliff (Fig. 13). Even though the megabreccias onlap the slope and are part of the LST, they are not interpreted to have been deposited in response to subaerial erosion (see Carbonate breccia triggering mechanisms). They appear to be related to continued seismic activity along the normal fault. The composition of the breccia clasts suggest that they were derived primarily from the margin and back-margin of sequences UK-1, UK-2, and UK-3. The back-cutting trend that was initiated in sequence UK-2 time progressed to the point where erosion of previously buried sequences (UK-1 and UK-2) was occurring. In the basin, TST and HST deposits of UK-5 are composed of marls-carbonate mudstone (Fig. 6). Thick (over 250 m) HST marls-carbonate mudstone fills remnant fault-derived, mid-Cretaceous topogra-

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phy and completely buried the margin of sequences UK-1 and UK-2, marking the end of coarse carbonate sediment gravity flow deposition in the region. High carbonate sedimentation rates in basins have the potential to reduce platform-to-basin relief and to decrease the depositional slope angle (Harris, 1991). Background sedimentation from suspension resulted in the observed marls-carbonate mudstones. There are no slump or slide deposits in this sequence, but intraformational erosion surfaces (Fig. 6) suggest some redeposition. Matrix-poor, clast-supported megabreccias originated from erosion of the exhumed margins of sequences UK-1 and UK-2, which by this time, were completely lithified. They formed rock falls and debris falls at the base of a fault scarp. Cohesionless debris falls periodically swept megabreccias up to 500 m out into the basin. Deformed marls and carbonate mudstones occur at the leading edge of the debris falls, and are interpreted to have been triggered by them. Many of the debris falls are capped by thin (10 – 20 cm), graded intraclastic packstones, which may represent coarsegrained turbidites (Fig. 12) that were associated with the debris flows (Krause and Oldershaw, 1979). Movement along the fault continued into the Coniacian, and during early sequence UK-5 time, a high

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(at least 60 m), near-vertical, submarine cliff is interpreted to have developed. Portions of sequences UK-1 to UK-3 were exposed to massive erosion and backcutting (Fig. 14). A thick talus pile of breccias developed at the base of the cliff, and periodic debris falls transported the breccias out into the basin (Fig. 14). This is interpreted to represent an erosional bypass slope. Tectonic activity along the fault ceased in the Coniacian, ending carbonate sediment gravity flow deposition in the area. During the TST and HST, the low abundance of reworked deposits suggests that the depositional angle in the lower slope region was reduced as fault topography was buried. Deep-water Santonian mudstone eventually covered the entire study area. 5.6. Sequence UK-6 Following the deposition of sequence UK-5, the initiation of foreland basin tectonics resulted in major erosion in the study area that excavated over 800 m of Cenomanian through Santonian (sequences UK-1 to UK-5) strata (Figs. 1, 2, 3, 4 and 16). The erosional topography was filled by siliciclastic turbidites with a westward transport direction (Van Hoorn, 1970).

Fig. 16. Photograph of a truncation surface between the light gray carbonates of sequence UK-2 and siliciclastic turbidites of sequence UK-6. View is looking west (see Fig. 1). This unconformity truncates sequences UK-5 to UK-2, and part of UK-1. Lower Cretaceous rocks (LK) are exposed in the background. Width of the photograph in the foreground is about 250 m.

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6. Carbonate breccia triggering mechanisms Fault-related oversteepening and syndepositional seismic activity are interpreted to be the dominant triggering mechanism for the sediment gravity flow deposits observed on the slope. The lack of reefconstructing organisms and the clastic nature of margin sediments in many of these Upper Cretaceous sequences would have limited their ability to build the high slope angles observed in sequences UK-2 to UK5 without the assistance of syndepositional faulting (Kenter, 1990). A normal fault is interpreted to exist beneath the upper slope of sequence UK-1 and the margin of UK2 for several reasons. First, a probable Albian to Coniacian normal fault has been mapped in this region (Fig. 1). Second, dramatic increases in sediment thickness from the shelf (100 m total for sequences UK-1 to UK-3) to the basin (total of 600 m for UK-1 to UK-3) have been observed to occur over short lateral distances (less than 300 m) across this fault. Finally, Albian to Coniacian breccias occur on the hanging wall block of the fault and are absent on the footwall block. Syndepositional growth along the fault is interpreted to have been an important control on creating and maintaining a steep, unstable slope, creating accommodation over the hanging wall, and promoting slope instability via oversteepening and seismic activity. Furthermore, seismic activity is interpreted to have been the primary triggering mechanism for the abundant gravity flow deposits in sequences UK-1 to UK-5 (Simo´, 1986; Drzewiecki and Simo´, 2000). Seismic activity may have also played a role in the timing of the emplacement of the matrix-rich, polymictic breccias during sequence UK-2. These deposits are concentrated in certain stratigraphic horizons (Figs. 1, 3 and 4), suggesting that episodic seismic activity may have directly triggered the collapse of the UK-2 sequence margin and initiated debris flows. Relative changes in sea level were probably not the primary control on megabreccia and megaconglomerate emplacement for several reasons. First, these lithologies occur in both lowstand (sequences UK-2 and UK-5) and highstand (sequence UK-1, UK-2, UK-3 and UK-4) systems tracts. Second, the slope appears to have been continually reworked from the late Cenomanian through Coniacian, and redeposited

sediments do not occur in discrete intervals that can be related to relative sea level positions. Third, the frequency of megabreccia/megaconglomerate occurrence is greater than the frequency of sequence boundary development on the shelf. Finally, there are many more megabreccia and megaconglomerate units than there are sea level cycles on the Haq et al. (1987) curve for certain ages (in particular, the late Cenomanian and late Coniacian), and it is not possible to associate individual units with individual sea level cycles. Differential subsidence between the shelf and basin, erosion and changing fluid pore pressures during relative sea level falls, and differential compaction probably also contributed to the development of a steep, unstable slope and the initiation of debris flows. However, it is not possible to isolate the individual contributions of each of these processes. Regional tectonic activity may have also played a role in the erosion and initiation of siliciclastic turbidity currents in the Santonian. Reactivation of the fault may have been responsible for major erosion in this vicinity, as foreland basin tectonics were initiated at this time (Puigdefabregas and Souquet, 1986).

7. Discussion and comparison to other carbonate slopes Evolution of the Pyrenean slope environment responded primarily to the changes in tectonic regime. Wrench tectonics resulted in the first phase of slope evolution. Extension not only established a steep depositional topography, but fault activity provided a triggering mechanism for margin collapse and breccia emplacement. A fault with a normal component of movement most likely controlled the distribution and composition of slope sediments, and seismic activity initiated the transport of coarse margin- and slope-derived material onto the lower slope during deposition of sequences UK-1 to early UK-5. Relative tectonic quiescence and high sedimentation rates during the later portion of UK-5 (phase 2), resulted in burial of fault-related topography. Finally, as foreland basin tectonics ensued, the study area became a site of major erosion and turbidite deposition (sequence UK-6, phase 3).

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Tectonics and sediment composition were major factors in determining the slope angle and types of gravity flow processes. Carbonate mud-rich deposits like those on the slopes of UK-1 to UK-5 are unstable at slopes of >5° (Kenter, 1990), so submarine slides, slumps, and slope-derived debris flows were common as long as a relatively steep depositional slope was maintained. Coarse, margin-derived debris flows were initiated by syndepositional tectonic activity in sequences UK-2, UK-4, and the early part of UK-5. Erosional modification of the lower slope most likely led to an instability that propagated upslope through time, as the slope adjusted to a more stable gradient. Slumping and downslope transport of material on the lower slope may have led to an increase in gradient higher up on the slope. This oversteepening, in turn, could have promoted instability, collapse, and bankward erosion. Descriptions of carbonate slopes that contain megabreccias are common in the geologic literature (Table 1) and provide a useful comparison for better understanding the depositional processes and controls that operated on Cretaceous slopes in the Pyrenean basin. There are three main types of (mega) conglomerates and (mega) breccias reported for carbonate platforms based on clast origin and depositional processes (Table 1). They include: (1) conglomerates/breccias with clasts derived exclusively from slope or other deep-water environments (Type 1), (2) conglomerates/ breccias with clasts derived from shallow (shelf and margin) environments mixed with clasts and/or matrix from deep (slope) environments (Type 2), and (3) matrix-poor conglomerates/breccias derived almost exclusively from shallow-water settings (Type 3). Type 1 and 2 breccias can be either matrix- or clastsupported, and are usually interpreted to be deposited from debris flows. Type 3 breccias are always clastsupported, and are interpreted to be the result of rock falls, avalanches, and debris falls (Table 2). Type 1 breccias typically form sheet-like deposits restricted to the slope and basin, and are interpreted to be the result of sliding and slumping on the slope that develop into non-erosive, mud-rich debris flows. Type 2 deposits are deposited by debris flows that resulted from catastrophic collapse of the margin. They have locally channelized bases and incorporate slope material during transport. Type 3 breccias originate by erosion of a steep, cemented margin or scarp, and essentially form a talus pile at the foot of that scarp.

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These conclusions, drawn from the literature, are consistent with observations made from the Pyrenean platforms. Sequences UK-1, UK-2, and UK-3 all contain slope-derived debris flow, slump, and slide deposits that have a sheet-like morphology and nonerosional bases. Soft-sediment deformation of clasts suggests that they were not fully lithified at the time of transport. Matrix-supported megaconglomerates (Type 1) are interpreted to represent mud-rich, unconfined debris flows. Sequences UK-2 and UK-4 contain channelized and sheet-like megabreccias with polymictic clasts (Type 2). The high concentration of angular, lithified clasts was responsible for the basal erosion associated with these deposits. The UK-5 sequence contains matrix-poor, polymictic megabreccias (Type 3), which are interpreted to be the result of erosion and deposition of rock falls and debris falls along a steep submarine erosional scarp. Type 2 carbonate megabreccias typically bypassed the slope and were deposited in the toe-of-slope and basin environments (Table 1). Once initiated, debris flows can be maintained for great distances across the entire slope before they ‘‘freeze’’ in the low angle toeof-slope environment (Lowe, 1979; Enos and Moore, 1983). However, in settings where the slope is steep and grain-rich (i.e. Devonian of Australia, Permian Lamar Formation of West Texas, Cretaceous of Mexico, and Miocene of Spain; Table 1), debris flow deposits are also preserved on the slopes. This implies that sediment type provides a fundamental control on the support and transport mechanisms of debris flows and that grain-rich slopes lack mud that provides the necessary buoyancy to allow debris flows to travel long distances over gentle slopes. Topographic irregularities on the slope also play an important role in controlling the lateral distance that sediment gravity flows can travel in a basin (Prather et al., 1999). Gravity flow deposits on platforms with erosional slopes differ from those reported for bypass or depositional slopes. Erosional slopes are characterized by an abundance of Type 3 deposits relative to other megabreccia types (Table 1). These margin-derived breccias often abut the scarp, a phenomenon which is attributed to the fact that Type 3 breccias are the result of rock falls, avalanches, and debris falls, which lack a buoyancy mechanism and are not able to transport material great distances (Lowe, 1979; Enos and Moore, 1983). These generalizations are consistent

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with the observations made for matrix-poor, polymictic megabreccias of sequence UK-5. Carbonate megabreccias can be triggered by a number of mechanisms (Table 1), including earthquakes, tsunamis, relative falls in sea level, undercutting of the margin by bottom currents, oversteepening of a platform margin, differential compaction, and even bolide impacts (e.g. Warme et al., 1991; Sandberg and Warme, 1993). Although a relative fall in sea level and accompanying subaerial erosion or decrease in slope pore pressures are interpreted to be the triggering mechanism for many breccia deposits (Table 1), the use of carbonate breccias to indicate sea level fall and lowstand deposition is not justified (Yose, 1991a,b; McLean and Mountjoy, 1993). Seismic activity is the most common triggering mechanism cited in Table 1, and for many platforms, the mechanism is unclear. Relationships between megabreccia deposition and syndepositional faulting in the Pyrenean basin clearly demonstrate that tectonics played an important role in slope failure and the initiation of margin-derived megabreccias in that region. However, slumping of slope material appears to have occurred continually on the lower slope, and not all of the resulting slumps, slides, and slope-derived debris flow deposits may be a result of seismic activity. The measured 12° slope angle was probably too steep for the stability of mud-rich sediments, resulting in continual reworking. Although seismicity was probably the primary triggering mechanism for the margin-derived megabreccias in this study, sea level fluctuations, oversteepening of the margin, and differential compaction between slope and margin sediments probably played secondary roles. Turbidites are commonly reported as a major component of the slope, toe-of-slope, and basin environments (Table 1). In the modern Bahamas, turbidity currents appear to be the primary process responsible for transporting shallow-water debris into the deep basins (Mullins and Neumann, 1979; Schlager and Chermak, 1979; Mullins, 1983, 1985; Mullins et al., 1984; Grammer and Ginsburg, 1992; Grammer et al., 1993). However, with the possible exception of the graded lithoclastic packstone caps on debris falls in sequence UK-5, turbidites have not been identified in the Pyrenean slope deposits. One possible explanation is that they have not been recognized because classic Bouma sequences were not preserved. A second and more likely possibility is that the part of the slope

presently exposed in the outcrop was too steep for the deposition of turbidites, and that they occur in the deeper basin setting (presently unexposed). Turbidity currents can be maintained as long as the energy lost to friction is compensated by gravity (Middleton and Hampton, 1973, 1976), and thus, they can be transported over slopes of less then 0.5° (Stow, 1986). The Cretaceous Pyrenean platform has a measurable slope of up to 12°, and turbidites are not likely to have been deposited. Siliciclastic turbidites do occur in sequence UK-6, but by this time, the shallow platforms have backstepped about 35 km, and the study area represents a deeper, basinal setting. Gravity flow deposits may be the combined result of several different transport mechanisms that either occur at the same time in different parts of the flow, or that vary temporally as the flow evolves (Lowe, 1979). Cook and Taylor (1977) and Cook (1979) have identified a close lateral association between submarine slump and debris flow deposits. The edges of slump deposits are commonly brecciated, and these authors suggest that continued brecciation would result in the total transformation of slumps into debris flows. A similar relationship was observed in slope deposits of sequences UK-1 and UK-2. Some submarine slumps and slides transform downslope into matrix-rich debris flow deposits containing contorted, elongate clasts that look like fragmented slope beds. Furthermore, turbiditic carbonate sands can occur immediately above debris flow deposits, and represent a temporal or lateral transition in gravity flow process of a single depositional event (Cook et al., 1972; Hopkins, 1977; Cook and Taylor, 1977; Krause and Oldershaw, 1979; Cook, 1979; Mullins, 1985; James and Stevens, 1986; Eberli, 1987; Lawson, 1989). Similar sediments were not recognized above the debris flow deposits of sequences UK-1 to UK-4 as a result of the steep depositional slope. Graded caps were identified above the debris fall deposits of sequence UK-5, but they are composed of granuleand sand-sized lithoclasts in addition to carbonate sand. This reflects the diminishing depositional slope in the Coniacian to Santonian, as the lower slope and basin were being filled. The role that sediment type played in slope stability has already been discussed. Unstable, mud-rich, lower slope sediments are extensively reworked when compared to the grain-rich upper slope. Undisturbed

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lower slope beds make up only about 10% of the sediment by volume in the lower slope. Perpetual reworking of these deposits resulted in continual erosive sculpting of the slope, and probably had a profound effect on the depositional angle and morphology of the slope at any given time. Sediment type has also been important in determining which sediment gravity flow processes operated on the slope. Mud was an important component of the sediment gravity flows that occurred during the Cenomanian through early Coniacian, and resulted in debris flows (both slope-derived and polymictic). Mud was not a significant component of the late Coniacian sediment gravity flows that occurred during sequence UK-5. The resulting processes tended to be matrix-poor rock falls and debris falls.

8. Conclusions The southern margin of the Pyrenean basin contains a variety of Cenomanian through Santonian gravity flow deposits and hemipelagic sediments, which are related to three phases of tectonic evolution. The phases include: (1) wrenching/rifting of the basin, with a gentle ramp (sequence UK-1) developing into a tectonically steepened carbonate platform (sequences UK-2, UK-3, UK-4, and the beginning of UK-5); (2) burial of a high-relief, fault-induced shelf to basin morphology during a period of relative tectonic quiescence (upper part of sequence UK-5); and (3) the excavation of a deep east – west trending scarp (channel?) and subsequent deposition of siliciclastic turbidites (sequence UK-6) resulting from the initiation of compressional tectonics. Sediment gravity flows resulted in three types of carbonate megaconglomerate/megabreccia deposits. Type 1 breccias are matrix-supported, slope-derived megaconglomerates interpreted as mud-rich debris flows that originated as submarine slides and slumps on a relatively steep platform slope. Type 2 are clastsupported, matrix-rich, polymictic megabreccias interpreted as debris flow deposits resulting from margin collapse, bypass of the upper slope, and deposition as channelized or sheet-like bodies on the lower slope. Type 3 are clast-supported, matrixpoor, polymictic megabreccias interpreted as a talus apron at the foot of a steep scarp, deposited by rock

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falls and debris falls. Type 3 breccias may preserve significant amounts of interclastic porosity and potentially form good hydrocarbon reservoirs. The first phase of platform evolution is characterized by a slope composed of hemipelagic sediments interbedded with slide and slump deposits and Type 1 megaconglomerates. As the slope progressively steepened, Type 2 megabreccias became more common. Type 3 megabreccias are common close to the fault scarp. The burial phase of platform evolution is characterized by Type 3 megabreccias interbedded with hemipelagic (some slumped) sediments. Renewed erosion and subsequent deposition of siliciclastic turbidites characterize the final phase of slope evolution. Sediment gravity flow processes are often interrelated on the slope. Debris flows originating at the platform margin appear to have initiated slumping and sliding on the upper and lower slope. Some submarine slides and slumps can be traced downslope into debris flow deposits. Debris fall deposits of sequence UK-5 contain graded lithoclastic packstone caps that may represent coarse calcareous turbidites associated with the debris fall events. Finally, debris flows on the lower slope may result in instability and erosion of the upper slope and margin regions, initiating margin-derived debris flows. Sediment type, and in particular, the presence or absence of carbonate mud, has a profound effect on the depositional processes that operate on a slope. Mud-rich carbonate slopes become unstable at lower angles, and are subject to higher amounts of reworking than grain-rich slopes. The triggering mechanism for the sediment gravity flow deposits appears to have been seismic activity associated with a normal fault that lies beneath the platform margin. Breccias are confined to a position basinward of the fault, suggesting that their origin and distribution are related to topographic relief associated with the fault. Megabreccias are common in both lowstand and highstand systems tracts and do not appear to be related to the relative position of sea level.

Acknowledgements Financial support for this project was provided by the National Science Foundation (Grant #EAR9315724), ARCO Oil and Gas, the Geological Society

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of America, and the Department of Geology and Geophysics, University of Wisconsin-Madison. This research benefited from discussions with John Booler, Alan Carroll, Bob Dott, Bruce Fouke, Kate Soriano, Phil Freiberg, Gary Gianniny, Irene Gomez-Perez, Kent Kirkby, Dave Mohrig, Lloyd Pray, Bill Raatz, and Debora Rodrigues de Miranda. Journal reviewers Finn Surlyk and Jan-Henk van Konijnenburg provided insightful reviews that improved the manuscript.

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