Carbonate slide deposits in the Middle Jurassic of Portugal

Sedimentary Geology, 57 (1988) 59-73 59 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Carbonate slide deposits in the Middle Jurassic of Portugal

M A R T I N R. G I B L I N G and C H A R L E S J. S T U A R T

Department of Geology, Dalhousie University, Halifax, N.S. B3H 3J5 (Canada) Unocal Science and Technology, P.O. Box 76, Brea, CA 92621 (U.S.A.)

(Received April 22, 1986; revised and accepted October 1, 1987)

Abstract

Gibling, M.R. and Stuart, C.J., 1988. Carbonate slide deposits in the Middle Jurassic of Portugal. Sediment. Geol., 57: 59-73

A Bajocian to Callovian carbonate sequence at Mareta Beach, Sagres, Portugal, contains slide deposits laid down on the southern continental slope of the Iberian Meseta. Translational slide deposits form nine discrete zones up to 6 m

thick. Intercalated hard and soft beds in lobate slide masses were deformed into megascopic recumbent folds, isolated

fold noses, and imbricate slabs, in a partially coherent to incoherent manner. A prominent lineation on planar and folded surfaces is due to microfold and microfault sets which formed while the beds were semi-consolidated.

Bioturbated omission surfaces overlie some slide deposits suggesting that slides incorporated near-surface sediments and formed topographically high sea floor features. Early partial cementation of the hard beds may have contributed to sliding by increasing pore pressure. A rotational slide scar closely associated with the deformed zones and low-angle truncation surfaces beneath some zones suggest that rotational failure may have initiated the translational slides. A southerly paleoslope toward the Tethys Ocean is indicated by orientations of folds, bedding-plane lineations, and slide scars.

Introduction

Submarine continental slopes are prominent sites of downslope movement, due, in part, to their inclination and to the high fluid content of surficial sediments. For example, about 40% of the submarine slopes of the Mississippi delta in the 10-200 m depth range show features of instability (Prior and Coleman, 1984). Such instability can damage drilling platforms, rigs, pipelines and communication cables (Heezen and Drake, 1964; Prior and Coleman, 1982). The lateral dimensions, surface appearance, and gross internal structure of modern slopes can be studied from seismic pro- files and side-scan sonar, but their inaccessibility hampers studies of depositional mechanisms and sediment distribution. Consequently, ancient slope deposits provide a more detailed source of infor- mation on the vertical dimensions, detailed internal structure, composition (modified by diagene-

0037-0738/88/$03.50 © 1988 Elsevier Science Publishers B.V.

sis), and depositional processes of ancient mass- transport deposits.

In this paper we describe a Middle Jurassic carbonate sequence in the Algarve, southern Portugal, that contains well-exposed bedding structures interpreted as translational slide deposits. Our principal objectives are to describe the stratigraphic setting and geometry of the structures present, and interpret the influence of sediment composition and diagenesis on the mechanism of sliding.

Geolo~eal setling

Mareta Beach section

The Algarve region (Fig. 1) lies on the southern margin of the Iberian Meseta, and formed part of the Tethys Ocean during the Mesozoic. Triassic redbeds resting unconformably on Hercynian

Jm

i Jm

f

~STUDY AREA;

c a k de Silo Vicelte

x x x Measured Section

./" Fault

Ju ~ Upper Jurassic

Jm ~ Middle Jurassic

JL ~--~ Lower Jurassic ( t i a~c )

IKm I !

Praia Tunel

Ponta ¢ Sagres

MARETA BEACH

/

N

Fig. 1. Location map for the Mareta Beach section, Sagres. 1-4 indicate subsections. General geology from the Vila do Bispo sheet of the Geologic map of Portugal (Manuppella et al., 1972).

basement are overlain by a predominantly carbonate sequence of Jurassic age. Jurassic strata are unconformably overlain by Cretaceous carbonate and terrigenous rocks (Rocha, 1976; Ribeiro et al., 1979). Stratigraphic ages of the Jurassic System are interpreted from Tethyan am-

Fig. 2. View of Mareta Beach section. Subsections are num-

bered 1 - 4 . b = breccia bed, r = reefal limestone, s = slide

deposits, d = disconformity at top of section.

monite assemblages (Rocha, 1976). The Jurassic stratigraphic sequence at Mareta Beach near Sages (Figs. 1 and 2) contains 135 m of Upper Bajocian to Callovian strata interpreted as continental shelf and slope deposits. The present study draws heavily on Rocha's report on the stratigraphy and fauna.

The Mareta Beach section (Fig. 3) consists of the four subsections shown in Figs. t and 2. The strata dip southeastward at angles of 5-15 o, and although faults are shown on the 1 : 50,000 map of the area (Manuppella et al., 1972), biostratigraphic data suggest that displacements are minor. The base of the section is faulted against Kim- meridgian-Oxfordian strata to the west, and is overlain disconformably by probable Middle Oxfordian strata to the east. Based on field evi- dence, subsections 1 and 2 (Fig. 3) are believed to belong to the same lithostratigraphic and chro- nostratigraphic level (see also Rocha, 1976, fig. 2.8).

The lower 12 m of the section consists of regularly bedded calcarenite and coarse calcilutite, with

Z j<C

,,J

0

Z

;1:> ~O

O

; ' Z

~G

n~

OXFORDIAN

DISCONFORMITY?

FAULT -- OS

UPPER UNIT (subsection 4)

SLIDE DEPOSITS FAO °s 0j

FAULTS INTRUDED BY

BASALT

MIDDLE UNIT (subsection 3)

10Msq

LO ER U.,T SUBSECT,ONS 3,*

SUBSECTION 2 SUBSECTION 1

E ~ DOLOMITE ~ CALCARENITE

CALC. SHALE ~ LIMESTONE CONGLOMERATE

CALCILUTITE ~ CORALLINE LIMESTONE

CONTORTED OS = OMISSION SURFACE BEDDING

Fig. 3. Stratigraphic columns of subsections 1 - 4 of the Maxeta

Beach section.

locally abundant oolitic beds and Zoophycos ichnofossils. Massive "reefal" limestone in the lower unit (subsection 2) contains abundant corals in growth position. The irregular upper surface of the limestone probably formed by solution during subaerial exposure (karst development). Lime- stone breccia occurs on the flank of the reefal limestone. This and a bed of breccia more than a meter thick in subsection 1 probably formed during subaerial or submarine erosion of the reef.

The middle unit of the section, about 55 m thick, consists of calcareous and weakly laminated, bluish-gray to brown shale with a few beds of fine

61

calcarenite up to 20 cm thick. These strata appear to rest directly on reefal limestone in subsection 3 (Fig. 3). Two basaltic dikes, 10-40 cm thick, occur in the upper part of the middle unit.

The upper unit of the section, about 68 m thick, consists of regularly bedded calcilutite composed of resistant and recessive (marly) layers interbedded on the scale of a few tens of centimeters. Deformed beds, interpreted as slide deposits, are a distinctive feature of the lower part of this unit. The upper part consists of undeformed beds of limestone with few recessive layers.

The Mareta Beach section represents deposition in shelf to slope or basinal environments. Reefal limestone of the lower unit was deposited in shallow water with water depths increasing upward in the overlying beds containing common Zoophycos traces. The middle unit may consist of basinal or lower slope deposits. The upper unit forms an upward transition from slope to shelf with slide beds in the slope-deposited part of the sequence.

Biota

The measured sequence contains a varied biota including ammonites, foraminifers, nannoplank- ton, and ichnofossils. The lower unit is char- acterized by an abundance of mesoscopic skeletal benthos and bioturbation, including Zoophycos. The middle and upper units are rich in the bivalve Bositra buchi, with little skeletal benthos or bioturbation. Ammonites, belemnites, and vascular plant fragments occur throughout, with cephalo- pods less abundant upward. A benthonic for- aminiferal assemblage dominated by Spirillina tenuissima occurs in the lower and upper units and suggests water depths on the order of 50 m or less, whereas agglutinated foraminifera are more abundant in the middle unit and suggest water depths of 200 m or more (Stare, 1985).

Lithology of slide deposits

Rock types

The interval containing the deformed zones consists of interbedded "hard" and "soft" beds of calcilutite, which is porcellaneous to finely crystal-

line. Deformed zones contain 20-80% of hard

beds which do not differ appreciably from the

beds in the intervening undeformed strata. Hard

beds are light gray to pale yellow (2.5Y 7 / 2 - 4 on

the Munsell colour chart), up to 30 cm thick, and

massive to weakly laminated. Soft beds are light

olive gray (5Y 6/2) , up to 1 m thick, and com-

monly form thin partings between hard beds. They

are massive and conchoidally weathered or weakly

laminated due to colour bands and layers of bi-

valve fragments. Graded beds and sole structures

were not observed. Both bed types contain a simi-

lar megascopic fauna, composed of Bositra buchi (more abundant in the softer beds), vascular plant

fragments up to 1 cm long, and a few ammonites,

belemnites, gastropods and fish. Bioturbation is

rare and Zoophycos was not observed.

Thin-sections show that the beds contain frag-

mented bivalves, calcareous and agglutinated for-

aminifera, algae, echinoderms, and a few fish

bones. Peloids, both true faecal pellets and micri-

tized skeletal grains up to 200 /~m in diameter,

form 10-50% of the rocks with 70-/~m dolomite

rhombs in a few samples and rare silt-sized quartz

grains. The finer matrix consists of micrite with

sparite cement in coarser bands. Minor amounts

of pyrite and limonite occur as irregular seams

and patches and as distinct cubes or spherical

(framboidal?) clumps. Chert is present as a minor

cement. The soft beds commonly contain more

peloids, dolomite rhombs, skeletal, and quartz

grains than the hard beds.

From X-ray diffraction analysis (Table 1), hard

beds contain abundant calcite and minor quartz,

whereas soft beds also contain appreciable quartz

and dolomite and trace quantities of illitic clay.

Soft beds also contain more insoluble residue and

TABLE 1

Semiquantitative percentages of minerals slide deposits, Mareta Beach (the method was followed)

in four translational of Cook et al., 1975,

Hard strata % (n = 7)

Soft strata % (n = 2)

Calcite Dolomite Quartz lllite

98-100 0 0- 2 0

~-95 0 - 5 5

trace

'FABLE 2

Geochemical composition of four translational slide deposit~, Mareta Beach [Total carbonate ~s calculated ~. ~:alciuln carbonate from a double-titration method (procedure B of Grimaldi et al., 1966}. X-ray diffraction analysis indicates minor dolomite in one soft sample. Insoluble residue is calculated by difference after ignition in a Leco induction furnace]

Hard strata % Soft strata % (n = 7) (n = 2)

Carbonate carbon mean 10.9 9.2 range 10.4-11.5 9.03- 9.36

Organic carbon mean I).00 0.24 range 0.14- 0.34

Total carbonate mean 91.2 76.6 range 86.4-96.0 75.3 -78.0

Insoluble residue mean 8.8 23.1 range 4.0-13.6 21.7 -24.6

organic ca rbon than hard beds (Table 2). The

mineralogy interpreted f rom both geochemical and

X-ray diffraction analysis is similar, a l though the

total carbonate content differs by about 10%,

probably due to the poor resolution of minor

minerals and amorphous material in the latter

method.

Three beds about 20 cm thick with abundant

b ioturbat ion were noted in contor ted parts of the

upper unit (Fig. 4). One bed directly overlies

deformed zone 2, t runcat ing underlying folds. The

bed is laminated and contains the highest propor-

tion of skeletal grains and peloids (mainly micri-

tized shell fragments) noted in any thin-section.

Limonite rims on peloids impart a distinctly red

tint to the rock. Dolomi te rhombs are rare, and

quartz grains and chert form about 1% of the

rock. A second b io turbated bed overlies deformed

zone 3 but is not red. A third bed (also red) occurs

10 m above zone 10.

Origin of the sediments

The strata conta ining deformed intervals were

deposited primarily by pelagic and hemipelagic settling of fine-grained carbonate and minor sili-

ciclastic and planktonic bioclastic materials. Win- nowed bands of coarser carbonate particles ce-

15

I0

DEFORMED FOLD SLIDE ZONES AXES SCARS

,o / 9

8 /

6 j, 5 \

4 / 3 J "

2

VECTOR MEANS (ALL DATA)

LINEATIONS

DIP

? /

Fig. 4. Detailed stratigraphic section of deformed intervals (slide deposits) in the upper unit of the Mareta Beach section, and linear features measured in the deformed beds.

63

mented by calcite spar indicate intermittent cur- rents, although features indicative of turbidity- current deposition (graded bedding, Bouma se- quences, sole markings, etc.) are absent. The bulk of the preserved biota is planktonic, and apart from benthonic foraminifera, shelly benthos are rare. These sediments resemble the periplatform oozes of modem carbonate slopes (Schlager and James, 1978) where carbonate mud derived from adjacent platforms settles into deeper water with minor admixture of siliciclastic sediment.

The differences between the harder and softer beds in the Mareta Beach section are subtle and can be explained most simply by a variable influx of hemipelagic carbonate sediment. When less carbonate was supplied, beds enriched in terrigenous material were formed, while the reduced sedimentation rate promoted an increased propor- tion of skeletal grains and peloids. Microcrystal- line calcite cement in these rocks may have originated partly from recrystallization of original aragonitic ooze derived from the platform (James, 1983, p. 337). The presence of organic matter and clay may have inhibited cementation of the softer beds.

Ferruginous, bioturbated bedding surfaces are interpreted as omission surfaces (Bromley, 1975) and represent periods when the net accumulation of detrital sediment was minimal. Beds underlying omission surfaces commonly contain winnowed grains and ferruginous material, and are bioturbated. However, the beds at Mareta Beach probably are not "hardgrounds" because they are even-bedded and grains within the beds are not truncated by the bioturbation (burrows rather than borings). Diffuse ferruginous patches suggest partial cementation of these surfaces. Beaudoin (1977, p. 145) noted the presence of calcarenite layers on top of synsedimentary deformed zones in the sub- alpine Mesozoic, and suggested that they rep- resented turbidite deposits generated by the downslope movement.

Slide deposits

General features of slide deposits

Shear failure occurs by horizontal displacement along discrete flat-lying planes (glides or transla-

64

tional slides) or curved planes of rotation (slumps

or rotational slides) (Nardin et al., 1979; Hansen,

1984; Gawthorpe and Clemmey, 1985). Shear

planes within translational slides cause variable

amounts of internal deformation, whereas rota-

tional slide blocks may be undeformed. Deforma-

tion structures within slide masses include folds,

microfaults, lineations, and poorly defined par-

tings; sediment behavior is elastic. There is a close

relationship between the two slide types, with

rotational slides commonly passing downslope into

translational slides in which lateral translation

greatly exceeds vertical displacement (Prior and

Coleman, 1984, pp. 429-430). Translational slide

deposits commonly occur in marine strata, whereas

rotational slide deposits are less common. The

term "slump" has been used loosely in the litera-

ture and is avoided here.

Deformed zones

Ten zones in the upper part of the Mareta

Beach section show deformation of discrete

packets of strata in association with planar shear

surfaces (Fig. 4). Most of these are interpreted as

translational slide deposits. The zones are up to 6

m thick averaging several meters. They are not

randomly distributed through the section but oc-

cur in four groups (Fig. 4): zones 1, 2-3, 4, and

5-10. Most zones extend laterally as far as the

outcrop can be traced (Fig. 5), but zones 8-10

pass laterally into undeformed strata (Fig. 6) and

f- i

I

. . . . . . . . . ::2~,~.~ /

Fig. 6. Slide zones g -10. Zone 8 (3 m thick) thins and the scale of folding decreases to the left toward the late-stage fault. Zone 9 (1 m thick) includes a distinct fold nose which passes to the left into undeformed strata. Zone 10 thins from 1.5 m off the right side of the photo, to () in the middle. Scale bar is 3 m.

zone 1 thins laterally against a high-angle, discor-

dant base. Each deformed interval is bounded by

abrupt planar to slightly undulating upper and

lower contacts. Folds are truncated by these

bounding surfaces (Fig. 7), except for the top of

zone 1 which is gradational. The intercalation of

intensely deformed and relatively undeformed

strata in a mildly tilted and faulted section sug-

gests that the deformed zones formed by synsedi-

mentary processes. Two zones are overlain by

beds interpreted as omission surfaces (Fig. 4).

Zone 5 consists of two packets of strata inter-

preted as rotational slide deposits.

Fig. 5. Translational slide deposits (zones 1 and 2). Note the intercalation of deformed and undeformed strata and the

imbricate appearance of both deformed zones. Zone 2 is overlain by a distinct ferruginous bed. Scale bar is 3 m.

Fig. 7. Abrupt contacts of slide zone 6 in which the upper contact truncates a recumbent fold. Note thickening of beds

into the nose of the fold.

Translational sfide deposits

General features. The character is t ics of de fo rmed

zones in te rpre ted as t rans la t iona l s l ide depos i t s

TABLE 3

Characteristics of translational slide deposits in the Mareta Beach section

Number of slide zones

Thickness

Vertical distribution

Lateral extent

Basal surfaces

Upper surfaces

Intervening strata

Lithology

Biota

Coherence of zones

Megaseopic folds

Megascopic fractures

1.0-5.75 m, average 2.65 m, uni- form laterally.

Present through 53 m of strata; grouped.

50-100 m+transverse and parallel to the direction of sliding; three zones pass laterally into undisturbed beds, lobate form and sub- units probable.

Abrupt, planar; underlying strata truncated locally.

Abrupt, planar; truncate folds in slide zones, two zones bounded by a bioturbated and/or ferruginous bed (omission surface).

Undisturbed, lithologically similar.

Fine to coarse caleilutite, pale yellow/gray, in intercalated hard and soft beds. 75-95% calcite, minor dolomite, quartz, iUite and limonite. Wackestone (skeletal grains and peloids in micrite). Weakly laminated to massive.

Planktonic bivalves (Bositra buchi), calcareous and agglutinated benthonic foraminifera, ammonites, belenmites, rare fragments of algae, echinoderms and fish. Nan- nopiankton, dinoflagellates, wood fragments. Bioturbation rare.

Semi-coherent to incoherent.

Upright to recumbent, folded masses up to 2 m thick; similar- style folds predominate; balled-up strata.

High- and low-angled (thrust) faults; joints and fissures.

TABLE 3 (continued)

65

Wrinkle lineation

Other features

Oriented structures (measured)

Paleoslope indicators

Microfold and microfault sets on planar and folded surfaces, commonly two sets at right angles.

Imbricate sheets, slid masses in topographic lows, parting in soft beds, slickensides.

Fold axes and isolate fold noses, wrinkle lineation, thrust faults.

Fold axis orientation and sense of rotation, lineation, imbricate sheets, associated rotational slide scars, regional setting.

are summar ized in Tab le 3. The zones range f rom

semi-coherent to incoheren t (Dzulyns ld and Wal -

ton, 1965, p. 191; Corbe t t , 1973). Semi-coherent

zones have unde rgone folding with only minor

d i s loca t ion and b r e a k d o w n of s t ra ta so that the

or iginal b e d d i n g s t ructure is still c lear ly visible

(Fig. 8). However , some zones re ta in their be dd ing

even though the or ig inal s t ra t ig raphic sequence is

indis t inc t (Fig. 9). Incoheren t zones consis t of

lent icular , l ayered r emnan t s r emou lded in to ho-

mogeneous masses (Fig. 10).

Folds. Mesoscop ic folds are the mos t s t r iking fea-

ture of t r ans la t iona l sl ide depos i t s at M a r e t a

Beach. U p t i g h t th rough inc l ined to recumbent

folds are present . Beds c o m m o n l y th icken at fold

hinges (Fig. 11), a l though concent r ic folds are also

present . The hinges of m a n y folds show ext reme

th ickening and a t endency to b reak up in to a

chaot ic mass of ro ta t ed remnants . These resemble

the rolls, ba l l ed -up s t ra ta and s lump bal ls de-

scr ibed by numerous au thors (e.g. Crowell , 1957;

G r a n t - M a c k i e and Lowry, 1964; Scott, 1966;

Spreng, 1967). D i s h a r m o n i c folds are c o m m o n in

the semi-coheren t zones (Fig. 9).

F o l d e d uni ts d i sp l ay several b roader - sca le fea-

tures. Semi-coheren t zones are charac te r ized by

b r o a d f lexures o r ien ted t ransverse ly to fold axes

wi th discrete fo lded masses loca ted in the hol lows

of the f lexures (Fig. 8). This re la t ionship suggests

that the con t inu i ty and in terna l s t ructure of sl ide

masses is at least pa r t ly con t ro l led b y the under ly-

ing topography . Several sl ide zones occur in close

66

Fig, 8. Slide zone 1. The strata are semi-coherent, with recognizable beds showing fold noses by the hammer (circle) and minor

dislocation due to folded sheets transposed along low-angle faults.

Fig. 9. Slide zone 8. The strata are semi-coherent and partially

broken-up into inclined, folded masses. The zone passes into

undeformed strata to the left. Folds to the right of the person

are concentric, with uptight to inclined fold axes.

Fig. 11. Detail of recumbent folds in shde zone 1, with strong

thickening at the hinges and dislocation of beds along low-an-

gle faults. Hammer is 30 cm long.

Fig. 10. Slide zone 2. The strata are incoherent with isolated

remnants of folds in a deformed matrix. Some show faulted

contacts (lower left). Hammer is 30 em long,

Fig. 12. Slide zone 2. Isolated fold noses and extended limbs have been thrust over each other, with imbricate s labs dipping

to the left (westward); the slabs slide downward at the extreme

fight (eastward). Hammer is 30 cm long.

proximity (Fig. 6), which suggests that several

lobate slides were generated at the same locality.

Isolated and rotated fold masses show an im-

bricate structure in which intervening limbs were thinned and transposed. The inclined surfaces sep-

arating imbricate masses dip at right angles to the orientation of predominant fold axes but in op-

posing directions. This is shown in Fig. 12 where folded slabs appear to have been thrust upwards

over each other from the left (northwest) then slid

downwards to the right (southeast). An imbricate

structure is shown in both slide zones of Fig. 5 and in the inclined contorted layers of Fig. 9

where the imbrication appears to have resulted from downsliding.

Imbricate slabs in ancient slide deposits were

noted by Gregory (1969), Corbett (1973), Beau- doin (1977) and Le Doeuff (1977), and were in-

terpreted as thrust slices dipping upslope. Gregory

(1969) noted that the dip direction gave a sense of movement opposite to that suggested by other

criteria and the slabs may have slid downslope. Fold axes within each deformed zone are well

aligned. Their orientation ranges from N - S to

E - W with a N E - S W vector mean (Fig. 4). Folds are ubiquitous in slide deposits reported in the

literature, and are their principal diagnostic feature (see literature reviews in Potter and Pettijohn,

1977; and Allen, 1982). Slide deposits which lack folding are difficult to identify.

Lineation. Many flat-lying and folded bedding surfaces within the deformed zones show a

67

Fig. 14. Close-up of a fold nose in slide zone 8 (right side of Fig. 6). Bedding surfaces through 1 m of strata show a consistent lineation (I) oblique to the fold axis. Scale is 16 cm long.

well-defined lineation, which is absent in the intervening undeformed strata. The str'Jcture imparts a

distinctive ridge-and-furrow form to the bedding

surfaces. The lineation at fold hinges is generally parallel to the fold axis (Fig. 13), although trans-

verse or oblique orientation was also observed

(Fig. 14). Figure 15 shows bedding surfaces at a fold axis where a finely spaced lineation parallel to the fold axis is cut by a widely spaced lineation

transverse to the axis. The structures show up to 5

m m of relief and 2 cm spacing and are discontinu-

ous over distances greater than 30 cm. The relationship among fold axes, secondary flexures, and

sets of lineation is shown diagrammatically in Fig. 16.

Fig. 13. Close-up of fold noses in slide zone 1 (Fig. 8). Individual recumbent folds, up to 40 cm in vertical extent, show a wrinkled appearance due to a lineation set parallel to the fold axes. The fold axes axe discontinuous along strike due to dislocation along low-angle faults. Hammer is 30 cm long.

Fig. 15. Close-up of a fold nose in sfide zone 3. A finely spaced "microfold" lineadon parallel to the fold axis (H) is cut by a widely spaced "microfault" lineation transverse to the fold axis (12). Scale is 16 cm long.

Calcite- filled fracture

cm Joint

2

,0 ~ ~ ~ ~ Lineation '~ 7 \ ~\~. .~---- ' - t '~--~ parallel to z ~ , ' , e o ~ , ~ \ ~ . ~//'/ fold axis

Lineation transverse \ r J .7 to fold axis

Fig. 16. Diagram of the hinge area of a fold in zone 1. The two sets of lineation are clearly related to the fold and its secondary flexure. Ticks on the lineations denote the steeper limb of the wrinkles.

Cross-sections of beds show that the l ineat ion

is due to symmetric to asymmetric microfolds of

laminae, a n d / o r regularly spaced microfaults

along which minu te blocks have rotated (Figs.

17-19). A gradat ion occurs between microfold

and microfault structures. The laminae in Fig. 17

mostly retained their cohesion but show slight

flexuring. Those in Fig. 18 exhibit adjacent flexures

that separated due to th inn ing of the limbs.

Harmonic flexuring of sets of laminae up to 0.5

Fig, 18. Section through a fold nose. Darker laminae separated into stacked, isolated flexures (S) oriented sub-parallel to the axial plane.

cm thick produced stacks of isolated fold noses,

which, in cross-section, locally give the appearance

of layering normal to the fold hinge. In Fig. 19,

more rigid blocks rotated along regularly spaced

flexures (microfaults?) parallel to the fold axis and

1 - 2 m m apart. Downth row across flexures is con-

sistently toward the hingeline. Laminae extend

down the f l e xu r e / f a u l t surfaces so that up thrown

and downth rown slabs remain connected. These

structures are p r omi ne n t at the contacts between

laminae bu t die out wi thin them.

Fig. 17. Section through the fold nose diagrammed in Fig. 16. The core of the fold is softer material than the hard rim. Folded laminae thicken into the fold nose but generally re- mained cohesive during sliding, deforming along small flexures. A set of calcite-filled fractures cuts the fold.

! > i i ¸

Fig. 19. Section though a fold nose. Laminae broke into very small slabs which rotated along "microfautts" with downthrow toward the fold axis. Lens cap is 5 cm in diameter.

The lineation on flat bedding surfaces is generally parallel to the fold axes in all of the slide zones in which they were observed (Fig. 4). The

combined vector means for both lineation and fold axes are closely parallel, suggesting that the lineation formed contemporaneously with large- scale folding. Directions of downthrow were not recorded because they were difficult to discern due to the symmetry of most structures. However, downthrow directions show a clear relationship to extension around fold hinges and secondary flexures of curvilinear hinges (Figs. 16 and 19).

Lineations and microfault sets have b e e n reported from ancient slide deposits (e.g., Spreng, 1967; Gregory, 1969; Thomson, 1973). Where measured, they are commonly parallel to fold axes. Directions of downthrow of microfaults are interpreted as downslope, although the sense of throw is complex in highly deformed units. Kent (1945) observed a closely spaced regular fracture set parallel to fold axes and an irregular set normal to fold axes, and suggested that they originated during flexure of the beds. Our observations are in accord with these studies, and the structures are taken to indicate intrastratal slip of laminae during sliding shortly after deposition. Similar structures elsewhere can reasonably be attributed to slip during differential compaction (Bradley, 1931). The lineation is thus a distinctive and important sedimentary structure that differs both in appearance and origin from wrinkle marks (Allen, 1985) and primary current lineation, but resembles tectonic lineation in that it formed by ductile behavior after burial. The structure has not commonly been recorded in the literature on slide deposits, and it may have been overlooked or interpreted as tectonic lineation.

Other structures. Beds in deformed zones that were poorly consolidated during sliding show a weakly developed parting with irregular orientation that resembles the "slump cleavage" of Corbett (1973) and Le Doeuff (1977). These structures are interpreted as shear surfaces formed during sliding.

Several structural features cut folds and lineations. Faults are common within the deformed zones and are closely associated with fold deformation. Sets of low-angle (< 30 °) fault planes

69

strike northeasterly, sub-parallel to fold axes, and represent surfaces of dislocation along which de-

tached folds were emplaced. High-angle fault planes show varied orientation and sense-of-throw within a single zone, although they generally strike northeasterly. Throws are a few tens of centimeters where measurable. Large-scale faults with throws on the order of meters also occur; their time of origin is not known but some could be early post-depositional structures.

Sets of joints and calcite-filled fractures are common (Fig. 17). Slickensides at fold hinges are both parallel and transverse to fold axes, being distinguished from lineation by their grooved and polished appearance.

Rotational slide deposits

Deformed zone 5 (Fig. 4) contains two packets of strata that failed along discrete, inclined surfaces with minimal internal disruption of bedding. The packets are interpreted as rotational slide deposits and their characteristics are summarized in Table 4. Principal slide surfaces underlie strata at least 1.5 m thick, and are associated with smaller, con-

cave-upward surfaces that separate a series of

TABLE 4

Characteristics of rotational slide deposits in the Mareta Beach section

Number of slide z o n e s

Thickness

Lateral extent

Basal surfaces

Upper surfaces

Internal deformation

Paleoslope indicator

One, with two major slide blocks containing smaller blocks.

1.5 m

15 m+ in direction of sliding, 5 m in transverse direction.

Truncate underlying beds; original dip of the major surfaces 14 ° (now 28°), minor surfaces locally steeper; concave- and convex-upward.

Planar, slides difficult to dis- tinguish from the overlying strata.

Minor.

Dip direction of detachment surface.

70

Fig. 20. Rotational slides in zone 5. View is along strike of a major failure surface, and shows a slide block with a series of nested lenses. A second major failure surface overlies the block and extends to the upper left of the photo. Hammer is 30 cm long.

nested slide lenses up to 50 cm thick (Fig. 20). The

lenses cannot be correlated readily with in situ strata, but they were probably displaced at least a few tens of centimetres. The blocks rotated only a

few degrees and it is difficult to tell whether their

tops were eroded. The major slip surfaces dip 14 ° southeast with a north-northeasterly strike after correction for tectonic dip. In sections parallel to the strike (Fig. 21), slip surfaces are highly irregu-

lar to concave upward and appear to die out laterally within a few meters, suggesting that these

deposits consist of lobate packets of strata. Rotational slide deposits in ancient marine de-

posits have been documented by Grant-Mackie

and Lowry (1964), Laird (1968), and Kennedy and Juignet (1974). I n t r a fo rma t iona l truncation surfaces are common in continental slope deposits

(e.g. McIlreath, 1977; Davies, 1977) and have been

interpreted as gravity slide planes, although rotated blocks are not associated with them.

Paleosiope determination

The use of translational and rotational shde deposits as paleoslope indicators is based on the

assumption that they represent downslope movements due to gravitational instability, although, as noted by Lajoie (1972), movement can be oblique

to the slope dip. Six features of the Mareta Beach slide deposits are considered indicators of paleoslope direction:

(1) Fold-axis orientation. The alignment of fold

axes within each zone (Fig. 4) indicates that sliding generated regular folds within both semi-

coherent and incoherent zones. Fold axes display

a relatively consistent northeasterly orientation through seven zones and 50 m of strata, which we interpret as the paleocontour direction. Most out-

crops include limited three-dimensional exposure which minimized observational bias due to outcrop orientation.

(2) Sense of rotation of folds. Fold asymmetry was used in some semi-coherent zones to indicate the downslope direction; for example, slide zone 1

shows a strong southward sense of rotation (Fig. 11). This criterion, however, is difficult to use in

Fig. 21. View downdip of major failure surface of Fig. 20. The small-scale surfaces are irregular and laterally discontinuous. Hammer is 30 cm long.

less coherent zones in which fold noses were de- tached and rotated. Most of these deformed zones

show no obvious sense of rotation with each

"downslope" nose balanced by an "upslope" nose (Fig. 10). The sense of rotation is also difficult to apply quantitatively.

(3) Lineation. Lineation on bedding surfaces is generally sub-parallel to the fold axes (Fig. 4), which constitutes a useful confirmation of slope orientation. Its presence could indicate sliding in strata which have not been folded. Lineation is commonly irregular or oblique to fold axes in highly deformed zones, although the direction of downthrow is meaningful where it can be dis-

cerned. (4) Slab imbrication. Slide slabs locally dip in

opposite directions within the same zone, depend- ing on whether thrusting or downsliding was oper- ative. However, the strike orientation of these slabs is a probable indicator of paleocontour direction. The strike of contacts between slabs or transposed folds are essentially low-angle faults which also indicate paleocontour orientations. At Mareta Beach, the strike of slide slabs suggests an east-west paleocontour direction. Some imbricate slabs suggest southward transport.

(5) Rotational slide scars. Though rare, slide scars provide the least equivocal downslope indicator based on the dip of truncation surfaces. At Mareta Beach, slide scars in zone 5 suggest an east-southeast slope dip direction.

(6) Regional considerations. In southern Portugal, the Iberian Meseta probably formed a low landmass north of Sagres during the Jurassic. Depositional surfaces within Middle Jurassic (Cal- lovian) strata at Mareta Beach probably dipped to the south away from this landmass. Consequently, indicators of southerly or southeasterly transport of Callovian slide deposits at Mareta Beach are reasonable. Local slope topography may explain some of the deviation from the regionally inferred paleoslope.

Conditions for sliding

The structural-sedimentary features of the translational slide deposits show clearly that the strata were semi-consolidated but not fully lithi-

71

fled at the time of sliding. Thickened hinges and

thinned limbs of many folds and isolated fold

noses show that the sediment was capable of adjusting to stress by flowage. Conversely, imbricate arrangement of sheets and fold noses, and associated low-angle faults, show that the sediment was consolidated enough to fracture. The microstructure of beds is especially instructive: the lineation shows a close association of microfolds and microfaults with plastic deformation of faulted blocks. The beds thus responded locally with both ductile and brittle behavior, but nowhere did the deformed zones break into breccia. Per- haps this indicates insufficient consolidation or a short distance of transport.

Although the distinction between hard and soft beds is based on their resistance to weathering in outcrop, their different styles of deformation in slide zones suggest original differences in geotech- nical properties. Hard beds generally formed cohesive masses while soft beds developed "slump cleavage" or broke down into homogeneous, incoherent lobes. The coherency of hard beds suggests that the sediments comprising them were partially cemented before sliding, in contrast to soft beds which were uncemented.

Soft-sediment folds in most deformed zones at Mareta Beach are sharply bounded by truncation surfaces. This suggests that only the upper few meters of sediment were involved in sliding. The two zones capped by omission surfaces may indicate reduced (or no) sedimentation over a topographically high pile of slide deposits. Zone 1 shows a gradational top and may have formed by intrastratal sliding along a decollement surface. Sliding was probably rapid in the former cases, but may have been a slow, downslope creep in the latter case, as inferred for some surficial slides in the Beaufort Sea (Hill et al., 1982).

The section demonstrates a close association between translational and rotational sliding, as deformed zones 6-10 are closely underlain by the rotational slides of zone 5, and the basal surfaces of several translational slides truncate the underlying strata. Some translational slides may have started as rotational failures (Gawthorpe and Clemmey, 1985), and this may account for their lobate form and their relationship to the underly-

72

ing topography. The grouping of deformed zones may reflect a series of retrogressive failures such as those documented by Faure et al. (1983).

Implications

Slope failure in modem environments results from numerous causes, including sediment load-

ing, gas generation due to geochemical processes,

oversteepening of slopes, loading by surface waves,

water-level changes, and earthquakes (Prior and

Coleman, 1982, 1984). Some of these factors de- crease shear strength whereas others increase shear

stress, combining to exceed the sediment shear

strength. Partial cementation of hard beds, especially those associated with omission surfaces or

hardgrounds, probably increases shear strength. During cementation, pore fluids in beds like these

might have been forced into adjacent uncemented beds decreasing their shear strength and increasing their tendency to shear. These mechanisms

and differences in shear strength may explain why

slides were so common in the middle unit of the Mareta Beach section. Seismic shocks caused by

sea-floor spreading of the adjacent North Atlantic

and transform faulting in the Azores-Gibral tar Zone (Sheridan et al., 1983), may have been an

additional mechanism initiating slides in the Sagres

area.

Conclusions

The Mareta Beach section contains interbedded hard and soft carbonate layers that were incorporated into translational and rotational slide deposits up to 6 m thick. Slide units contain soft-

sediment folds, microfold and microfault lineation, slump cleavage, faults, and imbricate slabs,

which, coupled with differences in composition of the hard and soft layers, suggest that the hard beds were partially consolidated at the time of

sliding. Such differences in coherency, if associated with pore fluid localization in the softer layers, may have promoted sliding on the inclined surface of the continental slope. The beds ap- parently were insufficiently consolidated to break up into breccia.

Sliding mainly involved the upper few meters

of sediment and was probably rapid, although slower, intrastratal sliding is probable for one zone. Although the relationships are unclear, the translational slides may have been initiated by rotational failure on th slope. Mesoscale structures

in the deposits indicate the regional southward paleoslope.

Acknowledgements

We thank Felix Gradstein, Beert Stam, Phil Hill, Bob Hickman, and two anonymous reviewers

for reading the manuscript, R.B. Rocha for val-

uable discussion, and Jim Berggen and Phil Jahnke

for assistance in the field. We also thank Doug Meggison for drafting, Norma Keeping for typing, and Martha Dupfisea and S.M. Parikh for techni-

cal analysis. Financial support from the Natural Science and Engineering Research Council of Canada (Grant A-8437) and from Unocal Corpo-

ration is gratefully acknowledged.

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Documents

Carbonate slide deposits in the Middle Jurassic of Portugal