Stratigraphic significance of siliceous microfossils ...geologie.mnhn.fr/PDW/De Wever et al...

ELSEVIER Marine Micropaleontology 24 ( 1995) 287-305

Stratigraphic significance of siliceous microfossils collected during NAUTIPERC dives (off Peru, 5”-6”s)

P. De Wever a, J. Bourgois a, J.-P. Caulet b, E. Fourtanier ‘, J. Barron d, P. Dumitrich ’ a CNRS-INSU, UniversitP Pierre et Marie Curie, DPpartement de Ge!ologie Sidimentaire, URA 1761, PalPontologie et Stratigraphie,

Case 117, 4 Place Jussieu, 75252 Paris Cedex 05, France b CNRS, M&urn National d’Histoire Naturelle, Paris, France

’ Diatom Collection, Department of Invertebrate Zoology and Geology, California Academy of Sciences, Golden Gate Park, San Francisco, CA 94118, USA

’ US Geological Survey, MS 915, Menlo Park, CA, 94025USA ’ Institut de Giologie et Paliontologie, UNIL, CH-1015 Lausanne, Switzerland

Received 2 August 1993; accepted after revision 4 August 1994

Abstract

The geological evolution of the northern Peru convergent margin can be traced using samples collected during deep-sea dives of the submersible Nuutile. In the Paita area (5”-6”S), the sedimentary sequence was intensively sampled along the main scarp of the middle slope area. It consists of Upper Miocene (7-9 Ma) to Pleistocene siltstone, sandstone and rare dolostone. The age distribution of these samples is the basis for a new geologic interpretation of the multichannel seismic line CDP3.

Siliceous microfossils (both diatoms and radiolarians) show influence of both cold and temperate waters (local species mixed with upwelling ones). Diatom assemblages studied from the NPl- 13 and NPl- 15 dives bear a strong resemblance to assemblages from the Pisco Formation of southern Peru.

Micropaleontological data from siliceous microfossils, provide evidence for two main unconformities, one is at the base of the Quatemary sequence and the other corresponds to a hiatus of 1 Myr, separating the Upper Miocene (7-8 Ma) sediments from uppermost Miocene (5-6 Ma) sediments.

During the past 400 kyr, a wide rollover fold developed in the middle slope area associated with a major seaward dipping detachment fault. A catastrophic debris avalanche occurred as the result of an oversteepening of the landward flank of the rollover fold. The gravity failure of the slope, recognized by SeaBEAM and hydrosweep mapping, displaced enough material to produce a destructive tsunami which occurred 13.8 f 2.7 kyr ago.

1. Introduction

During the “Nazca Plate Project” ( 1972-1980), three transverse multichannel seismic profiles were shot in an area of the landward slope of the Peru Trench located between 5” and 12%. According to the interpretation of the northern line CDP3, located off Paita (Fig. 1) , Sheperd and Moberly ( 1981) concluded that the upper slope is deformed by block faulting and that

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the middle and the lower slopes exhibit the diffraction pattern of an accretionary prism. The subsequent reprocessing of the profile by Miller et al. (1986) allowed tracing to the middle slope area, the industry- well geology projected from the shelf. It was proposed that the upper slope consists of block-faulted metamorphic continental rock covered by Cretaceous to Quatemary sediments extending westward to the seaward edge of the upper slope. As a consequence, they

288 P. De Wever et al. /Marine Micropaleontology 24 (1995) 287-305

MIDDLE SLOPE F

8 “0 $ 9

UPPER SLOPE

Fig. 1. Location of dives performed during the NAUTIPERC cruise off Peru. Line with barbs indicates Peru Trench convergence zone in upper left box. The main area discussed in this paper was surveyed during the Leg 1 of the NAUTIPERC cruise (25 dives), off Paita. The long arrow shows direction of the subducting Nazca Plate convergence relative to the South America Plate, the numbers within the arrows are the convergence rates (in cm/year) determined by the SLR solution (Smith et al., 1990), the AM02 model (Minster and Jordan, 1978) and the NUVEL 1 model (DeMets et al., 1990)) respectively. The main body of the figure locates the dives performed in the Paita area. Map of SeaBEAM contours at 100 m interval and seismic line CDP3 crossing it (line with dashes and points, see also Fig. 7). The track-lines of the dives are reported by black lines and the corresponding dive number. Routes of dives 4, 27, 32, 35 and 36 am not strictly superposed but are here combined for convenience. The Upper Slope Scarp locates the western boundary of the upper slope; it coincides with a fault (F). The Middle Slope Scarp conforms to the limit of the detachment-surface which provided the avalanche debris. The Subduction Scarp (SS) bounds the Trench (7’) from the Lower Slope. Line with barbs indicates Peru Trench convergence zone.

inferred that the base of the sedimentary sequence, if not the basement, crops out along the scarp at the upper slope-middle slope boundary.

The knowledge of the structure of the middle slope area was greatly improved by SeaBEAM bathymetry acquired simultaneously with four multichannel seismic profiles shot perpendicular to the CDP3 line. The main morphological features which have been distin-

guished in the surveyed area are (Bourgois et al., 1986, 1988) :

( 1) a 400 to 700 m high scarp (Upper Slope Scarp, Figs. 1 and 2) which separates the upper slope from the middle slope. The seismic profile 02 clearly shows (Bourgois et al., 1988; Von Huene et al., 1989) that this scarp corresponds to a major normal fault relief (Bourgois et al., 1993) as evidenced by a series of

P. De Wever et al. /Marine Micropaleontology 24 (1995) 287-305 289

Fig. 2. Three-dimensional diagram showing the position of samples (in circles) along the dives (in squares) in the Paitaarea. Due to perspective the middle slope here appears relatively narrow. Some close samples are not reported due to the lack of space, their approximate position is nevertheless easy to infer. The 3D diagram is from a mesh net perspective diagram of SeaBEAM bathymetry (Bourgois et al., 1988)

disrupted reflectors dipping 45” seaward. This normal fault deeply penetrates into the continental margin down to 3000 m below seafloor.

(2) a 1000 to 1200 m high curved scarp (Middle Slope Scarp, Figs. 1 and 2)) was found 10 km seaward from the Upper Slope Scarp. The northern section of the Middle Slope Scarp trends E-W whereas the south- em section trends N-S. The northern section is located in the prolongation of one of the E-W trending canyons which characterizes the morphology of the upper slope. The morphology of the northern part of the middle slope is characterized by small valleys and ridges roughly paralleling the curvature of the Upper Scarp. This indicates that the structure of this region is con- trolled by mass movements.

(3) a 300 to 500 m high scarp (Subduction Scarp, Figs. 1 and 2)) is located at the base of the lower slope.

It bounds the trench landward and corresponds to the present-day morphological expression of the subduction front.

(4) between the Middle Slope Scat-p and the Sub- duction Scarp, the lower slope is characterized by a rough topography with closed highs and lows and represents the surface of a debris flow detached from the Middle Slope Scarp.

(5) isolated mounds on the flat floor of the trench indicate that the mass wasting travelled down to the trench axis.

Von Huene et al. (1989) have proposed that the Upper Slope Scarp and the Middle Slope Scarp bound a landward tilted block detached along a listric fault, and that the Middle Slope Scarp represents the boundary of an area voided by slumping. They emphasized that the detachment of the block (20 X 33 km) was


Fig. 3. Middle Slope Scarp exhibits a small graben in which active fluid venting is associated with clam colonies. During the dives numerous clams and associated colonies were discovered which are related to fluid vents (and often along structural fractures, see dive 6). Colonies arc most abundant along the northern section. The largest clam field was observed during dive 4 of leg 1 (Figs. 2 and 3) and was intensively surveyed during several dives of leg 2 (dives 27, 32, 35, 36) located at the junction between the southern section of the Middle Slope Scarp and the prolongation of an E-W trending canyon of the upper slope

catastrophic and produced the debris avalanche that covered the lower slope and extended across the trench axis. The volume of the displaced material is thought to be sufficient to produce a destructive tsunami (Von Huene et al., 1989; Bourgois et al., 1993).

The primary objectives of the deep-sea dive cam- paign conducted with the submersible Nuutile during the NAUTIPERC cruise (March-April 1991) off northern Peru includes geological observations and chemical studies of venting fluids at a convergent margin devoid of an accretionary prism and where subduction erosion (Scholl et al., 1980; Hussong and Wipperman, 1981; Aubouin et al., 1984; Von Huene and Culotta, 1989; Bourgois et al., 1990; Von Huene and Lallemand, 1990; Von Huene and Scholl, 1992) and massive subsidence are active (Kulm et al., 1984; Glacon and Bourgois,

1985; Kulm et al., 1988; Suess et al., 1988a, b; Von Huene et al., 1988; Resig, 1989).

The twenty one deep sea dives performed along the continental margin off Paita (Figs. 1 and 2) explored the three main scarps that characterize the morphology of the continental margin along the CDP3 profile and provide new insight into the geology of this area. We report hereafter the main results of the micropaleontological study based on the deep-sea submersible collection of samples. These data provide more detailed constraints on the geology of the continental margin and the trench and allow a new line-drawing interpretation of the CDP3 profile. The main unconformity along the upper and the middle slope areas assumed previously to be at the base of the Eocene section (Von Huene et al., 1989) corresponds to the unconformity


identified to the south at the base of the Pliocene to uppermost Miocene section (3-6 Ma in the Chiclayo canyon area (Sosson et al., 1994) and at the ODP Sites 683 and 679 drilled during Leg 112 (Von Huene et al., 1988; Bourgois et al., 1990)). Moreover, the late Pleis- tocene tectonic history can be retraced in detail (Bour- gois et al., 1993): during the past 400 ka, a broad rollover fold formed in the middle slope area. As the result of oversteepening of the seaward flank of the rollover fold, a major catastrophic debris avalanche occurred at 13.8 f 2.7 ka.

and radiolarian assemblages in dives NPl - 13 and NP l- 14.

(2) 300 to 400 m of massive siltstone, sandy mudstone and rare beds of sandstone. A latest Miocene- early Pliocene age (5-6 Ma) is documented by diatom, radiolarian and silicoflagellate assemblages in dives 10, 13 and 15.

2. Description of sites

2.1. The Middle Slope Scarp

Sixteen dives were performed along the Middle Slope Scarp: three along the E-W trending section, four along the N-S trending section, and eight in the elbow area, at the junction between the northern and southern sections of the scarp (Fig. 2). The Middle Slope Scarp appears to be composed of a succession of small cliffs exposing dominant mudstones and siltstones separated by steps covered with recent pelagic sediments. Scarps a few meters high exposing sandstones or dolostones are observed locally. In detail, the topography is com- plicated by a succession of small canyons separated by sharp ridges (Fig. 3). Outcrops are frequent along the steepest slopes. Screes are observed at the foot of small scarps. Generally, the freshness of the outcrops suggests that the failure occurred recently.

(3) 0 to 500 m of diatom and radiolarian bearing mudstones. They exhibit a maximum thickness along the E-W trending section of the Middle Slope Scarp. To the south, this formation wedges in between the Pliocene and the late Pleistocene sequences. A Quater- nary age (0.4-1.8 Ma) is documented by radiolarian and diatom assemblages, with the base of the Quater- nary constrained by the occurrence of the Lumprocyrtis nigriniae radiolarian assemblage which first appearance in the East Pacific occurred 1.4 to 1.5 Ma ago (Johnson and Nigrini, 1985).

(4) 50 to 75 m of pelagic diatom ooze unconformably overlies the older sequences. A late Pleistocene age, younger than 0.4 Ma, is based on the occurrence of Lamprocyrtis nigriniae (Johnson and Nigrini, 1985 ) and the last occurrence of Stylatractus uniuersus (Mor- ley and Shackleton, 1978).

The dominant siltstones are generally poorly con- solidated and are affected by a dense network of fissures and joints with various orientations. Their orientation has been measured directly from the submersible. Along the N-S trending section of the Middle Slope Scarp, the dominant directions are NO”, N40”-50”, N90“-loo”, and N120”. Along the’ E-W trending section the dominant directions are NO”-10”, N20”-30”, N90”-100” and N130”.

The two westernmost dives performed along the E- W trending section of the Middle Slope Scat-p show beds dipping seaward, in contrast with dippings inferred previously from the “landward tilted block” model (Von Huene et al., 1989). This suggests that the middle slope area is not a simple, rigid, tilted block but a wide anticline with a N-S trending axis within a separate block down-dropped toward the trench (Fig. 2). The anticline structure is overlain by the post 0.4 Ma pelagic sedimentary cover.

2.2. The upper slope scarp

Two deeply incised canyons cut through the Upper Slope Scarp at the upper-middle slope boundary. They have been the site of three dives of the NAUTIPERC cruise. Two main results of the geological survey are:

Microfossils analyses of 84 samples, collected along ( 1) In contrast with the Middle Slope Scarp area, the Middle Slope Scarp, constrain the geology. The the post 0.4 Ma pelagic sedimentary cover is very thick, reconstructed lithostratigraphic succession of the mid- leading to a smooth topography, indicating that the dle slope area is, from bottom to top: detachment fault surface is totally buried. A late Pleis-

( 1) 400 m of Upper Miocene mudstone and sand- tocene age, based on the occurrence of Lamprocyrtis stone which include: the early late Miocene (7-9 Ma), nigriniae and the last occurrence of Stylatractus univ- documented at the base of the sequence from diatom ersus is evidenced in dive 6.


(2) Active fluid vents occur along the Upper Slope scarp.

According to Von Huene et al., ( 1989)) the detachment along the Upper Slope Scat-p fault was catastrophic and would have occurred very shortly before (or after) the failure which affected the Middle Slope Scarp. Therefore, the Upper Slope Scarp should exhibit morphological characteristics similar to those of the Middle Slope Scarp.

A composite section can be assembled from samples collected on dives NPI - 13 (early-late Miocene to early Pliocene), - 14 (early and late Pliocene), and -4 (late Pliocene and Quaternary). Tables l-3 give the occurrence of characteristic and stratigraphically important diatoms in samples collected during these three dives.

The major detachment fault related to the Upper Slope Scarp (Bourgois et al., 1988; Von Huene et al., 1989) displaces the upper sequence 0.8 km down (Von Huene et al., 1989). Bourgois et al. ( 1993) proposed that the anticline which extends seaward across the middle slope area is linked to the detachment fault and thus would be a rollover fold. The time when the rollover began to form can be estimated. The detachment cuts the Quaternary sequence (0.4-l 5 Ma), as shown by MCS data (Von Huene et al., 1989). The detachment scarp was covered progressively during the past 0.4 Myr by pelagic sediments while the middle slope area subsided. Thus, we assume that the detachment and the related middle slope rollover fold started at 0.4 Ma.

3. Age of sediments

3.1. Diatom stratigraphy

Middle Slope Scarp The Middle Slope Scarp (MSS) off Paita was sam-

pled on dives NPl-4, -13, -14, -15, -16, -29, -30, -31, -32, -35, and -36. Diatomaceous sediments range in age from early-late Miocene to Quaternary.

Indurated claystone in samples NPl-13-2 through - 13-5 (Table 1) contains a diatom assemblage that is early late Miocene in age (ca. 8-8.7 Ma) and temperate in character rather than tropical, closely resembling age-equivalent assemblages from California. The first occurrence of Rossiella tatsunokuchiensis in sample NPl-13-6, however, is considerably older than its ear- liest Pliocene first occurrence in California (Barron, 1992) ; however, Yanagisawa (in press) also recorded an early late Miocene first occurrence for the species at DSDP Site 438 off Japan. It seems likely that R. tatsunokuchiensis evolved in warmer waters of the Pacific and later migrated into cooler regions of the middle latitude North Pacific. The occurrence of N. miocenica in the same sample (NPl- 13-7) with R. paleacea and T. grunowii is surprising, because in the eastern equatorial Pacific, N. miocenica appears slightly after (0. l- 0.3 Ma) the disappearance of R. paleacea and T. grunowii (Barron, 1992). This latest Miocene diatom assemblage is clearly more tropical in character than the underlying early late Miocene diatom assemblages. In fact all post-7 Ma diatom assemblages (Pliocene and Pleistocene) studied from the NAUTIPERC dives have a strong tropical character. The uppermost Miocene to lowermost Pliocene interval of about 3.6 to 6.5 Ma is missing in the dive NP l- l- 13 section and either is pres-

Table 1 Occurrence of characteristic and stratigraphically important diatoms in samples collected during dive 13

Epochs

P. De Wever et al. /Marine Micropaleontology 24 (1995) 287-305

Table 2 Occurrence of characteristic and stratigraphically important diatoms in samples collected during dive 15

293

Sampler

ent in the unsampled section between samples NPl-13- 7 and -13-8 (between depths of 3190 and 3265 m) or was removed at an unconformity. Part of this missing section is present in samples NPl-15lb through -15-4 of dive NPl-15 (Table 2) which are early Pliocene in age (3.2-5.1 Ma). This lower Pliocene section and the underlying upper Miocene section recovered on dive NPl-13 (Table 1) contain diatom assemblages that closely resemble and are correlative in age to diatom assemblages described from the Pisco Formation of southern Peru ( (Mertz, 1966; Fourtanier and Machare, 1988; Koizumi, 1992).

Samples NPl-15-6 and -15-7 are late Pliocene (ca. 1.8-2.3 Ma) based on the presence of Rhizosolenia praebergonii and the absence of Pseudoeunotia doliolus and Thalassiosira conuexa. The absence of Actin- ocyclus ellipticus f. lanceolatus and Nitzschia jouseae in sample NPl-15-5 also suggest a post-early Pliocene age for that sample, which is confirmed by radiolarians which suggest an age of 2.6 to 2.4 Ma.

Samples collected during dive NPl-4 range from latest Pliocene to middle Quaternary (ca. 1.0-2.0 Ma) based on the occurrences of Pseudoeunotia doliolus, Rhizosolenia praebergonii, R. matuyamai, and Nitz- schia fossilis, (Table 3). Like the upper Pliocene of dive NPl- 15 (Table 4), this section is characterized by abundant Chaetoceros spores and has a strong upwelling character.

Fig. 4 shows the ranges of stratigraphically important diatom species and references to tropical diatom zones

for the late Miocene to Pleistocene (Barron, 1992)) the approximate geologic age ranges of dive sections NPl- 13, -15, and -4 (Tables l-3) deduced from diatom biostratigraphy, and the apparent local stratigraphic ranges of selected diatom taxa.

Debrisjow (dives I, 2, 3) upper slope scarp (dives 5, 6, I7), subduction scarp (dives 8, 9, 10, 1 I, 12) trench (dive 7)

Table 4 shows the geologic ranges of all of the NAU- TIPERC samples studied for diatoms from the Paita area as well as from the Chiclayo Canyon and other areas to the south (NP2, Fig. 1) . Most of the samples from the Upper Slope Scarp, debris flow below the Middle Slope Scarp, the Subduction Scarp, and the trench area off Paita are Pleistocene in age based on the presence of the diatoms Pseudoeunotia doliolus and/ or Actin- optychus bipunctatus (Fig. 4) or silicoflagellate stratigraphy. The exceptions are sample NPl-3-2b from thedebrisflowandsamplesNPl-lo-l,NPl-lo-6,NPl- 1 l-2b, NPl-11-5, and NPl-12-4 from the Subduction Scarp which are latest Miocene to early Pliocene in age (see Table 4).

Samples taken from the Middle Slope Scarp, on the other hand, also include material from the lower part of the upper Miocene (parts of dives NPl- 13 and NPl- 14)) the uppermost Miocene (sample NPl-30- 1) , the lower Pliocene (parts of dives NPl-13, -14, -15, -16, and -36)) and the upper Pliocene (parts of dives NPl-

294

Table 3


Occurrence of characteristic and stratigraphically important diatoms in samples collected during dive 4

4, -15, and -30). The age distribution of these samples is the basis for drawing the geologic structure.

3.2. Radiolarian stratigraphy

Described radiolarian assemblages from the Eastern Pacific Ocean (Nigrini, 1968, Molina-Cruz, 1977, 1984) are considered as characteristic of upwelling areas (Nigrini and Caulet, 1992). Tropical forms are rare; but north Pacific species may be present (De Wever et al., 1990, 1992).

Radiolarian assemblages were studied in 95 samples collected during dives NP 1- 17. Samples were prepared according to the standard method described by Sanfi- lippo et al. (1985) but the material was sieved at 80 pm to eliminate clay aggregates. A second mesh of 50 pm was used for control. For each sample examined, semi-quantitative estimates of overall radiolarian abundance and preservation and specific abundances of stratigraphic markers were made (Table 5). Zonal assignments are a combination of the zonation com- piled by Morley ( 1985) for the Plio-Quaternary of the North Pacific, and that of Riedel and Sanfilippo ( 1978) for the Miocene (Fig. 5). Ages of biostratigraphically useful datums are given for both the Cande and Kent ( 1992) and Berggren et al. ( 1985) geomagneticpolar- ity time scales. The source of correlation of the indi- vidual datums to the magnetic polarity chrons is also given (Table 6).

3.3. Radiolarian occurrences

Radiolarian ages are in good agreement with the diatom stratigraphy (Table 5).

Debrisflow (Dives 1,2,3) : Rare and well-preserved radiolarians were found in all (but 5) sediment samples from Dives NP-1 to -3. Upwelling species constitute the major part of radiolarian assemblages. Many species common in North Pacific assemblages co-occur with the eastern tropical Pacific assemblages.

Upper Slope Scarp (Dioes 5, 6, 17): Radiolarians are rare and well-preserved in sediments from the Upper Slope Scarp. Some reworked specimens of late Miocene-Pliocene age were found in Sample NPl-5- 1. Taxa are similar to those found in the sediments from the Debris Flow.

Middle Slope Scarp (Diues 4, 13, 14, 15,16): Radi- olarian assemblages from this area are very rare to rare and well-preserved, however many samples were barren (Table 5). The oldest samples (late Miocene-late Pliocene) were found in that area. Late Miocene assemblages are mostly composed of representatives of Acti- nomma spp. group (Nigrini and Caulet, 1992)) Anthocyrtidium prolatum, Botryostrobus bramlettei, Diartus hughesi, Lamprocyclas hannai, and Sticho- corys peregrina. Some specimens of Cycladophora robusta were found in Sample NPl-4-18. One repre- sentative of Stichocorys delmontensis was found in Sample NPl-13-7. Very rare and probably reworked specimens of Diartus petterssoni were observed in Sample NPl-13-2.

Table 4 Geologic ranges of all of the Nautiperc samples studied for diatoms from the Paita area as well as the Chiclayo Canyon and other areas to the south (Leg 2). For each dive, the following information is given: dive number; number of samples examined; number of samples that are definitely Quatemary; number of samples that are probably Quatemary; number of samples that are barren or indeterminate in age; samples that are not Quatemruy: their number and their age

#samples Def. Prob. Barren Other Age [Dive examined Quat Quat. Ilndet. Sample Ma

Debris Flow - 1 9 7 1 PI 1 ___ 2 3 3_ 3 2 1 3-2b (4.5-3.2)

Unnar Slam SC 1

- _. _._ r_ _larp I I I 51 11 lj I

t . _ ._ . , \ _ . _ . _ ,

I I I I I it-u ItA G-3 fi\ I / I I I” ” I\_.” h.“,

j 111 71 31 1 121

2111_2b, 41 31 I 1 12-4

I 1 *

M liddle Slope Scarp / I 1 9, .n IUI n YI I I a . ,n n n n\ , 9-1 j\L.d-L.U) I 131 71 1 13-2 to-7 118.7-7.01

I _. .

I I I I I I 13-13 -R It!? Q-9 61

1 361 11 I I l 36-l ](3.6-2.6) LEG 2 southern I I


8-LI-‘L-LI- ‘S-LI-‘E-LI-‘P-SI-‘E-SI- ‘9I-PI-'9-PI-'4--EI-‘E-EI-‘9-ZI-‘S-ZI-‘6-II- ‘S-II-'91-II-‘99-01‘9-01-'E-OI-‘I-OI-‘6-6-‘9 -6-‘9-‘~S-6-‘9-‘ep-6-‘8-8-‘9E-8-‘E-L- '9-9-‘I-9-‘~I-j,-‘9-@-“,p-,,- ‘E-@‘q‘l?S-E- ‘8-Z-‘E-Z-‘OI-I-IdN :i?&I~MOI~O3 a~lS!lS!I~!aq]‘Uaueqa~aM saIdcues pauy~~xa @~aaas .pauodar s! say&30 aauasqe JO aauasald 6Iuo xuawfaads 1aeluF Ma3 hraa ~I!M uognIoss!p30 aa~%ap qlx e3o su8!s moqs sueyo!p~= (Jood) d !uog~uatu~w3 snoyqo qq~ uogtqowp aJeJapom 30 aauap!aa MOMS sueyzIo~peJ = (awapom) pq !uogeluaw2e.r3 ~oouiru Quo ql!M uognIoss!p 30 u%!s ou moqs sue~Io~pe~ (~00%) 9 :8uf~olIo3 aql uo paseq set a8eIqurasse ueyelo!peJ aql30 uo!iehJasaJd ‘(aptIs uo suam!x& 0s > ) areJ = a : (suauq3ads ooI-&) Ma3 = d : (apgs uo suauqaads r~os*)oI ) uouuuo3 = 3 :SMOIIO~ se passasse set aaUEpUnqe aseIqUIaS%? uy.IeIo!pE~ ‘~1-1 saaip ‘I asyu~ 3IEldLLnvN luoy saldwes II! UIauIu%!SSe a8e %UlInSaJ pue sue!.rEIo~peJ 30 saxIaJ.rnaao

s a1qe.t

‘(SgfjI ) ‘@ la UaJ%laa 30 a@as arup LlyIod a!]au%?em

aql Pm (Z661 ‘uoJ%I) a!ozoua3 ale? aql Jo3 Sauoz mole!p Iea!doJl 01 paleIauo3 slaymu a!qde.@lew mo)e!p ]ueuoduq 30 sa8ue~ ‘p 2~~

Dm -I m

Z

962.


Table 5

297


$ i 4

a

1

2

3

I

5

6

7

B

B

Radiolarian Biozonation

c4

I D. penultima

- ---_

L4A

D. antepenultimc

Range of stratigraphic markers

- J

Fig. 5. Correlation of radiolarian zones, and ranges of the stratigraphic markers to the geomagnetic polarity time scale of Cande and Kent ( 1992). Question marks are used when the age of the event is uncertain.

Subduction Scarp (Dives 8, 9, 10, 11, 12): Many sediment samples from this area are radiolarian-free. Recrystallized forms (Dictyoprora montgolfieri) of Paleogene age were found in sample NPl- 1 l- 11. Many species characteristic of upwelling area are common in these assemblages (particularly Phormostichoartus schneideri).

Trench (Dive 7) : Pleistocene radiolarians are very rare and well-preserved in the samples from Dive 7.

3.4. Age from silicojlagellates, ebridians and siliceous endoskeletal of dinoflagellates (actiniscaceae)

For age determination the warm-water silicoflagellate zonation of Bukry ( 1981) was applied.

Based on this zonation the following ages were estimated for the fifteen samples studied under optical microscope (Table 7) :


Table 6 Ages of Neogene radiolarian datum levels based on the Berggmn et al. ( 1985), and Cande and Kent ( 1992) (CK92) geomagnetic polarity time scales. LO = last occurrence; FO = first occurrence. Sources for ages of datum levels: ( 1) Hays and Shackleton (1976); (2) Morley (1985); (3) Johnson et al. ( 1989); (4) Spencer-Cervato et al. (1993)

Event Datum Berggren CK 92 (Ma) (Ma)

Source

Rl LO

R2 LO

R3 FO

R4 LO

R5 LO

R6 FO

R7 FO

R8 LO

R9 LO

RlO FO

RI1 LO

RI2 FO

R13 FO R14 LO

RI6 FO

R15 LO R17 LO

Stylatractus universus Lamprocvrtis neoheteroporos Lamprocyrtis nigriniae Eucvrtidium matuyamai Lamproqrtis heteroporos Eucvrtidium matuyamai Qcladophora dauisiana Stichocotys peregrina Theocotythium trachelium Lamprocyrtis neoheteroporos Stichocotys delmontensis Lamprocvrtis heteroporos Sphaeropyle langii Didymocyrtis penultima Stichocotys peregrina Diartus hughesi Diartus petterssom

0.425 0.45

0.61 0.64

0.8 0.85

0.98 1.05

1 1.07

1.8 1.9

2.7 2.8

2.8 2.9

2.8 2.9

2.9 3

3.4 3.55

4.4 4.6

4.4 4.6 6.4 6.8

6.8

7 11.1

7

7.2 11.3

1

2

4

2

2

2

4

2

2

3

4

4

2 2

4

4 4

NP l-6- 1 (argillaceous diatomite with Stephanopy- xis sp.) : The sample contains a warm-water assemblage belonging to the Mesocena quadrangula Zone and is, accordingly, Middle Quaternary in age. Interesting in this assemblage is the presence of the species Diste- phanus tenuis Bukry, known thus far only from the eastern equatorial Pacific, DSDP 503A, where it was considered to range from the Upper Miocene to the Lower Pliocene (Bukry, 1982). Since in sample NPl- 6- 1 this species reaches a frequency of 8% and silicoflagellates older than Middle Quaternary are absent we

infer that it is not reworked and that its range has to be extended up to the Middle Quaternary.

NPl-13-2: a cool-water silicoflagellate assemblage characterized by the abundance of Distephanus speculum s.1.

NPl-13-2b: The sample is a diatomite with Stephan- opyxis sp. The silicoflagellate assemblage is rich in Distaphunus speculum s.1. indicating a cool-water environment. The presence of rare fragments of P. apiculatu would suggest a Late Miocene age but the presence of the radiolarian species Tholospyris rhombus (Haeckel) that, according to Go11 ( 1972), has its first appearance in the upper Stichocorys peregrina Zone (Early Plio- cene) would indicate that the assemblage is not older than the Pliocene. We did not detect sign of reworking or contamination.

NPl-13-6: A cool-water assemblage with frequent Distephanus speculum, few D. boliviensis and rare Dic- tyocha ausonia.

NPl-13-8: The sample contains a cool-water assemblage dominated by the presence of Distephunus speculum. Notable is also the presence of remains of phaeodarian radiolaria.

4. Discussion

4.1. Ages distributions

Ages are plotted in Fig. 6. Most of the stratigraphic column is well represented except for two horizons. For one of them (Upper Pliocene, 2-3 Ma) we cannot be affirmative about the lacuna. Referring to the position of sample transectsNPl-13 and NPl-15 as well as transect NPl-14 on Fig. 2, it is clear from Tables 1, 2, 4 and 5 that no sediments dated between 6.3 and 5.1 Ma were sampled in this portion of the Middle Slope Scarp, thus strengthening the argument for an unconformity (see Fig. 6). The lack of sediments of that age probably corresponds to the unconformity which is known southward in the Chiclayo canyon (Sosson et al., 1994) where the Lower Pliocene overlies directly the Paleozoic basement. A similar unconformity is known on land where the Pliocene is unconformably resting on the Paleozoic. It also corresponds to the unconformity documented at sites 683 and 679 during leg ODP 112 (Von Huene et al., 1988).

300 P. De Wever et al. /Marine Micropaleontology 24 (1995) 2X7-305


3 iH z-1 C2A w o-

M 0

4 -> -JJ an:

C3 a

5- w

w 2 w

7- IH- 0 c4 0

i----H I

6

C4A w I- d

9 -I ix C6

0

Fig. 6. Stratigraphic distribution of samples dated by diatoms and radioltians) from NAUTIPERC Cruise 1 related to the geomagnetic polarity time scale of Berggren et al. ( 1985) ordered according to their origin (for location see Fig. 2).

4.2. Geology and evolution of the margin

According to the interpretation of the CDP3 line by Miller et al., ( 1986) including the geology projected from the shelf (Sheperd and Moberly, 1981) , sedimentary rocks of Eocene and Cretaceous age were sup- posed to crop out along the Upper Slope Scarp and the Middle Slope Scarp. The stratigraphy of the continental shelf was extended, based on onshore and offshore exploration south of the Talara oil field. Along the upper slope the top of the Eocene section was thought to be truncated by a submarine erosional unconformity covered by Neogene strata. The Oligocene to Miocene unit was proposed to thin and pinch out at the Upper

Slope-Middle Slope break. The inferred Paleocene and Cretaceous sequences were roughly traced down to the middle slope area. These interpretations should be reconsidered since the oldest sedimentary rock sampled at the southern toe of the Middle Slope Scarp is Upper Miocene in age (7-9 Ma) and since the main unconformity shown on the multichannel seismic lines 01 to 04 (Fig. 1) along the Upper and Middle Slope area and interpreted to be pre-Neogene in age by Von Huene et al. (1989) is herein identified to be the base of the Quaternary sediments.

The predetachment fault topography (Fig. 7) of the Upper and Middle Slope area was reconstructed (Von Huene et al., 1989) by matching the subsurface geol-


CDP 3

d8bris. avalanche. 11000 - 16500 yr

Fig. 7. Three-dimensional diagram showing the main tectonic features and rock age. Compare with Fig. 2. The 3D diagram is from a mesh net perspective diagram of SeaBEAM bathymetry (Bourgois et al., 1988). The thick line crossing the diagram refers to the CDP3 seismic line. The Upper Slope Scarp is covered by pelagic sediment, thus contrasting with the typical aspect of the Middle Slope Scarp. The slide debris traveled across the flat trench floor to the seaward slope and was subsequently carried toward the deformation front by plate convergence.

ogy and aligning the sea floor topography along the project the geology of the Middle Slope Scarp upslope, CDP3 line. A complementary constraint was obtained along the CDP3 line, as shown in Fig. 8. by aligning landward dipping reflectors cut by the The unconformity documented from the MCS lines detachment fault at about 5 km depth. The predetach- 01 to 04 is at the base of the Upper Pliocene to Quater- ment topography is well constrained and the 5” back nary (0.4 to 1.5-2 Ma) sequence. It probably repre- rotation of the landward flank of the rollover fold which sents a hiatus, as shown in Fig. 6. The unconformity extends throughout the Middle Slope area allows to extends along the middle slope and the upper slope

E

Rollover fold

km 4 6 b 8 B 7 7 8

CDP 3 DEPTH SECTION 8 9

Fig. 8. Line drawing of the seismic line CDP3 including the biostratigraphic data. The three main sites of dives are reported in square boxes (7, 8-12 on the Subduction Scarp SS, 14.13-16, 18-19.27.32.35-36: on Middle Slope Scarp MSS; and 5,6,17 on Upper Slope Scarp UXS. Note the unconformity within the Miocene.


areas from 25 to 75 km landward from the trench axis, during Oligocene and Miocene times. Therefore the to the slope-shelf break at 200 m water depth. This Peruvian margin would have suffered several kilome- unconformity would corresponds to the unconformity ters of subsidence during the past 5-6 Myr as previ- known on-shore at the base of the Tablazo Formation ously documented along profile CDPl off Lima (Von (De Vries, 1988). Huene et al., 1988; Bourgois et al., 1990).

The Quaternary-Upper Pliocene sequence overlies a Pliocene (3-5 Ma) sequence extensively sampled along the N-S trending section of the Middle Slope Scarp. The oldest age documented in the sequence is 5 Ma (sample NPl-15-1) as a maximum that allows to infer a hiatus of about 1.2 Ma with the underlying late Miocene sequence. This unconformity is assumed to be the main regional unconformity since it is documented extending from the Upper Slope to the Lower Slope area along the Chiclayo Canyon transect located 100 km to the south. Moreover, this unconformity is also known onshore at the base of the Hornillos For- mation in the Sechura Basin. In that area the uppermost Miocene to Pliocene post-dates an extensional tectonic phase which is widely recognized in northern Peru. Therefore we assume that the main unconformity which extend throughout the Upper Slope area is at the base of the Pliocene sequence instead of the Miocene as previously proposed.

Acknowledgements

The NAUTIPERC cruise was supported by IFREMER, INSU-CNRS and IST-Program, the Bundesminister- iumfir Forschung und Technologie (Bonn) and GEO- MAR. We thank the Captain and the crew of the R/V Nadir for their efficient work. Onshore studies were supported by CNRS-URA Paleontologie et Stratigra- phie and the French Minister-e des Affaires Etrangdres. We thank also the French Embassy and the Znstitut Francais des Etudes Andines (IFEA) in Lima for the help in getting the authorization to work off Peru.

Appendix: Species list of radiolarians

As a consequence, the upper Miocene sequence (7- 9 Ma) identified at the base of the N-S trending section of the Middle Slope Scarp during dives NPl-13 and - 14 does not extend upslope. We think that it is located in a graben structure older than the Pliocene as it is described on-shore in the Paita area and to the south in the Chiclayo Canyon area (Sosson et al., 1994).

Bibliographic references for radiolarian taxa used in this study precise the present concept of these species, they are given below. Descriptions and illustrations of the upwelling species may be found in Nigrini and Caulet ( 1992) :

Actinomma spp. group [in Nigrini and Caulet (1992)]:

The Late Cretaceous to Tertiary basin which is located along the Upper Slope area is a southward prolongation of the Talara basin known onshore north of the city of Talara. Since the Mancora Formation of Oligocene age represents the basal sequence of the Tumbes basin which develops north of 4% we assume that the Upper Slope basin along the CDP3 transect includes only deposits ranging from upper Cretaceous (Redondo, Montegrande, Ancha and Petacas Forma- tions) to upper Eocene (Verdun and Chira Forma- tions) unconformably overlying the metamorphic basement as in the Talara basin, south of 4%. Therefore the hiatus at the base of the Pliocene section which extends from late Eocene to early Pliocene (5-6 to 34 Ma) covered a long period of time (28-29 Myr) . We think that the area extending along the present day Upper Slope and Middle Slope was above sea level

Acrosphaera murrayana (Haeckel) Dictyophimus infabricatus Nigrini Eucyrtidium erythromystax Nigrini and Caulet Lamprocyclas hadros Nigrini and Caulet Lamprocyrtis nigriniae (Caulet) Lamprocyclas maritalis ventricosa Nigrini Lithostrobus sp. cf. hexagonalis Haeckel Phormostichoartus schneideri Nigrini and Caulet Phormostichoartus crust&a (Caulet) Plectacantha cremastoplegma Nigrini Pseudocubus warreni Go11 Pterocanium auritum Nigrini and Caulet Pterocorys minythorax Nigrini

Descriptions and illustrations of the north Pacific species may be found in the publications cited.

Eucyrtidium matuyamai Hays; Kling, 1973; Morley, 1985


Lumprocyclas heteroporos (Hays); Kling, 1973; Morley, 1985

Lamprocyclas neoheteroporos Kling; Kling, 1973; Morley, 1985

Pterocanium korotnevi (Dogel) ; Nigrini and Moore, 1979

Sphaeropyle langii Dreyer; Kling, 1973; Morley, 1985

Sphaeropyle robusta Kling; Kling, 1973; Morley, 1985

Descriptions and illustrations of the cosmopolitan species can be found in the publications cited.

Anthocyrtidium prolatum Nigrini and Caulet; Nigrini and Caulet, 1992

Botryostrobus brumlettei (Campbell and Clark) ; Caulet, 1979

Cycladophora davisiana Ehrenberg; Nigrini and Lombari, 1984

Cycludophoru robusta Lombari and Lazarus, 1988 Diartus hughesi (Campbell and Clark) ; Nigrini and

Lombari, 1984 Dictyoproru montgolfieri (Ehrenberg) ; Nigrini,

1977 Lumprocyclas hannai (Campbell and Clark);

Nigrini and Lombari, 1984 Phormostichoartus pitomorphus Caulet; Caulet,

1986 Stichocorys delmontensis (Campbell and Clark) ;

Nigrini and Lombari, 1984 Stichocorys peregrina (Riedel) ; Nigrini and Lom-

bari, 1984 Stylatractus uniuersus Hays; Nigrini and Lombari,

1984

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