Cenozoic marine sedimentation in the Sechura and Pisco basins, Peru

Palaeogeography, Palaeoclimatology, Palaeoecology, 77 (1990): 235-261 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Cenozoic marine sedimentation in the Sechura and Pisco basins, Peru R O B E R T B. D U N B A R 1, R I C H A R D C. M A R T Y 1, a n d P A U L A. B A K E R 2

1Department of Geology and Geophysics, Rice University, Houston, TX 77251 (U.S.A.) 2Department of Geology, Duke University, Durham, NC 27708 (U.S.A.) (Received September 22, 1988; revised and accepted July 31, 1989)

Abstract Dunbar, R. B., Marty, R. C. and Baker, P. A., 1990. Cenozoic marine sedimentation in the Sechura and Pisco basins, Peru. Palaeogeogr., Palaeoclimatol., Palaeoecol., 77:235 261. The central and northern Peruvian margin consists of a series of 8 paired forearc basins which may be separated into an inner set of shelf basins and a seaward set of slope basins. We have examined the Cenozoic stratigraphy of the onshore portions of the Sechura Basin (5-7°S) and Pisco Basin (13-16°S), two shelf basins which have accumulated marine sediment discontinuously since the mid to late Eocene. Cenozoic sediments in the Pisco Basin were deposited during at least three major transgressive cycles. Each sequence is preserved as a similar vertical progression of facies including coarse nearshore bioclastic conglomerates and sandstones grading upwards into sandy siltstones and mudstones, and capped by biogenic deposits including diatomites, diatomaceous mudstones, dolomitic horizons, and phosphate deposits. Stratigraphic nomenclature for the Pisco Basin has recently evolved; a stratigraphy presented here includes the Eocene Caballas Fm., upper Eocene Los Choros fm., upper Eocene to lowermost Oligocene Yumaque fm., uppermost Oligocene to middle Miocene Chilcatay fm., and upper Miocene to Pliocene Pisco Fm. Major hiatuses in the Pisco Basin span the Late Cretaceous to middle Eocene, early to late Oligocene, middle Miocene, and late Pliocene/Pleistocene to Recent. Cenozoic sediments of the Sechura Basin were deposited within at least 4 major transgressive cycles with hiatuses during the Paleocene to middle Eocene, Oligocene, early to middle Miocene, and late Miocene. Based on recent biostratigraphic studies, sediments enriched in biogenic components accumulated between about 40 36 Ma, 24 16 Ma, and 11- 3 Ma in the Pisco Basin and between 40 37 Ma and 8.5 4.5 Ma in the Sechura Basin. In both basins, the most diatomaceous sediments are restricted to the Late Eocene and Late Miocene through Pliocene. The temporal distribution of biogenic sediments suggests that high productivity conditions linked to coastal upwelling have occurred episodically since at least the Late Eocene. The occurrence of diatomites and phosphorites is diachronous between the Pisco and Sechura Basins and between the Peruvian forearc and other circum-Pacific Monterey Formation analogs, a reflection of the strong influence of local tectonism on sedimentation patterns. The volume of Neogene sediments along the Peruvian forearc is nearly twice that of the Monterey Fm.; despite basin-to-basin facies diachroneity, these deposits very likely contributed to fluctuations of the late Miocene carbon/CO 2 system by acting as large carbon sinks.

Introduction The continental margin of western South A m e r i c a is o n e o f t h e w o r l d ' s m a j o r c o n v e r g e n t plate boundaries. Although subduction tectonics and o r o g e n e s i s h a v e o c c u r r e d since at l e a s t t h e e a r l y M e s o z o i c ( H e l w i g , 1972; M e g a r d , 1973), m o d e r n f e a t u r e s o f t h e m a r g i n d e v e l o p e d 0031-0182/90/$03.50

mostly during Cenozoic time (Thornburg and K u l m , 1981; M o b e r l y e t al., 1982). E x t e n s i o n and subsidence, punctuated by p e r i o d s o f uplift, have produced a series of forearc basins which occur both onshore and offshore from southern Chile north to Ecuador. Relatively few Andean forearc sedimentary sequences have been studied in detail and both local

© 1990 Elsevier Science Publishers B.V.

236

stratigraphies and regional correlations are presently in a state of flux. Extreme coastal aridity coupled with Pliocene/Quaternary coastal uplift has produced superb onshore exposures of forearc sediments and structures along the Peruvian sector of the margin. Recent field studies in this area (DeMuizon and DeVries, 1985; Machar~, 1987; Machar~ et al., 1988b; Marty, 1989; papers collected in Dunbar and Baker, 1988) coupled with results from offshore drilling (Von Huene et al., 1987; Suess, Von Huene et al., 1988) are yielding a valuable record of Cenozoic mountain building, subsidence, and paleoclimates along the South American coastline. Cenozoic sedimentary units of the Peruvian forearc range from Eocene to Recent in age. The average sediment thickness in the basins is 3000-4000 m. The margin lies beneath the Pert] Current and is greatly influenced by cold water advected from the south and high productivity associated with coastal upwelling. The development of this eastern boundary current system has left a distinct record in forearc sediments, manifested by the occurrence of organic-rich shales and siltstones, diatomites, phosphates, and organic styles of diagenesis. The offshore portion of the Peruvian continental margin has been extensively surveyed in recent years by scientists from the University of Hawaii, Oregon State University, and the U.S. Geological Survey (Hussong et al., 1976; Coulbourn and Moberly, 1977; Shepherd and Moberly, 1981; Thornburg and Kulm, 1981; Von Huene et al., 1985; and others). ODP Leg 112 recently drilled ten sites on the Peruvian margin with principle objectives of defining the accretion/erosion, subsidence/uplift, and palaeoceanographic histories of the margin (Von Huene et al., 1987; Suess et al., 1987; Suess, Von Huene et al., 1988). The onshore forearc basins have received far less attention, but nonetheless comprise a major resource for studies of margin evolution. In this paper, we briefly describe the Cenozoic sediments of the East Pisco Basin (13-15°S, 75-76°W) and Sechura Basin (5-7°S, 80-81°W),

a . B . DUNBAR ET AL.

Peru, two of the best exposed forearc basins along the Latin American margin.

Methods and time scale conventions Most of our field work in the Sechura and Pisco basins was conducted during 1985-1987. Approximately 120 sections have been measured, described, and sampled. A summary of the measured sections is presented in appendices of Dunbar and Baker (1988), Marty (1989), and Stock (1989). Weight % biogenic opal contents are determined by time-series dissolution of samples in 0.1 N NaOH following procedures outlined by DeMaster (1979) with corrections for volcanic glass as outlined by Marty (1989). Organic carbon contents determined by Carlo-Erba CNS analyzer. Samples for K/Ar analysis were collected from pure ash horizons within the Pisco Basin. Fractions of 0.5-5.0 g of biotite (grain size 125-300 ~m) were recovered through sieving and repeated use of a Franz magnetic separator. K/Ar determinations were made at Oregon State University by Robert Duncan. Radiolarian checklists and photographs from representative Sechura and Pisco Basin sections may be found in Marty (1989). For this paper, we use the Neogene zonal age conventions of Barron et al. (1985), Paleogene radiolarian zonal age assignments according to Riedel and Sanfillipo (1978) as reported in Berggren et al. (1985), and Paleogene diatom zones as reported in Haq et al. (1987).

Geologic setting The continental margin of Peru consists of a series of ten Mesozoic and Cenozoic forearc basins separated and defined by three positive linear trends, the Upper Slope Ridge, Outer Shelf High, and Coastal Batholith (Fig.l; Couch and Whitsett, 1981; Thornburg and Kulm, 1981; Shepherd and Moberly, 1981). Thornburg and Kulm (1981) suggests that the Upper Slope Ridge is composed of deformed sedimentary units. The Coast Range-Outer Shelf High is a horst cored by metamorphic

CENOZOIC MARINE SEDIMENTATION

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and crystalline rocks (Caldas et al., 1980), and the Coastal Batholith is a complex of igneous rocks mostly emplaced during the Late Cretaceous-Eocene when arc magmatic activity was close to the location of the present coastline (Giletti and Day, 1968; Cobbing and Pitcher, 1972; Noble et al., 1979; Mukasa, 1986). Two sets of basins occur between these structural

highs. Thornburg and Kulm (1981) have defined those between the Upper Slope Ridge and Outer Shelf High as slope basins and those landward of the Outer Shelf High, including the East Pisco and Sechura basins, as shelf basins. The thickness of the Tertiary sediment infill within the forearc basins decreases from north to south with a maximum stratigraphic

238

thickness of 9000 m in the Talara Basin (Travis et al., 1976) and 2000m in the Pisco Basin (Machar6 and Fourtanier, 1987). Although the Talara Basin of northern Peru contains sediments of Cretaceous age (Bosworth, 1922; Travis et al., 1976), sedimentation in most of the basins began following Late Cretaceous/ Eocene compressive tectonism. Episodic orogenic events during the Cenozoic are most likely linked to variations in relative rates of convergence and subduction, direction of convergence, and angle of subduction (Helwig, 1973; Megard, 1972; Handschumacher, 1976; Pardo-Casas and Molnar, 1987). Following the initial synthesis and nomenclature of Steinmann (1929), McKee and Noble (1982) and Caldas (1983) describe three major phases of deformation influencing the Peruvian Andes. The ~'Peruvian" phase spanned the Late Cretaceous through Eocene; large-scale folding, intrusive activity, and uplift resulted in complete regression of the sea from the present-day forearc. The late Eocene-early Oligocene "Incaic" orogenic phase was the most important Cenozoic compressive event in the Andean cordillera, resulting in large-scale folding, crustal thickening, and transverse deformation along the H u a n c a b a m b a and Abancay deflections. The Miocene-Pliocene deformational period, termed the "Quechua" phase by Steinmann (1929), has been subdivided into three separate intervals of folding and volcanic activity (Megard, 1973; Noble et al., 1974; McKee and Noble, 1982). The Plioc e n e - R e c e n t period is characterized by continuing orogenesis with uplift of up to 3000 m in the Peruvian Andes and maintenance of vigorous volcanic activity in southern Peru (Caldas, 1983). In a recent kinematic analysis of the Peruvian forearc, Macher6 et al. (1988b) have recognized 4 principal compressive tectonic events at approximately 42, 28, 7, and 3 Ma. Machar6 et al. (1988b) characterize sedimentation in the forearc basins as limited by low overall subsidence rates (averaging <0.1 mm yr -1) during lengthy periods of tensional tectonism interrupted by short-lived compression/uplift events.

R . B . D U N B A R E T AL.

An important influence on Neogene tectonics and sedimentation within the Peruvian forearc is subduction of the aseismic Nazca Ridge. The Nazca Ridge is an area of topographically high, buoyant oceanic crust which is presently being subducted beneath the South American Plate adjacent to the Pisco Basin of southern Peru (Fig.l). The trend of the Nazca Ridge is oblique to the subduction vector and the locus of ridge subduction has moved southward along the Peruvian Coast since at least the late Miocene causing localized and time-transgressive uplift of the forearc. Reconstruction of the geometry of the subducted portion of the Nazca Ridge and its position through time (Pilger, 1981; Fig.2) shows that ridge subduction began in northern Peru about 9-10 m.y. ago. Marty et al. (1987) have attributed mild deformation and uplift at approximately 7-8 Ma in the Sechura Basin of northern Peru to the passage of the Nazca Ridge. Hsu (1988) has linked rapid late Pleisto-

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CENOZOIC MARINE SEDIMENTATION IN SECHURA AND PISCO BASINS, PERU

cene uplift in the southern part of the Pisco Basin (up to 0.47 m kyr-1 at Cerro E1 Huevo) and lower uplift rates in the northern portion ( < 0.1 m k y r - 1 at Bahia de la Independencia) to their respective positions over the leading- and trailing-edges of the subducting Nazca Ridge.

East Pisco Basin: geology and stratigraphy The East Pisco Basin lies at the juncture of several important structural trends (Figure 1). The Outer Shelf High and Coastal Batholith merge onshore with Paleozoic and Precambrian rocks of the Arequipa Massif. The Abancay Deflection lies north of the Paracas Peninsula and forms the boundary between the submerged and subaerial parts of the basin. The Nazca Ridge is presently being subducted beneath the onshore portion of the basin. The geology of the Pisco Basin has previously been described by Lisson (1898), Adams (1908), Petersen (1954), Newell (1956), Ruegg (1956), Mertz (1966), Balarezo et al. (1980), Machar~. (1987), and I N G E M M E T (unpublished preliminary geologic quad sheets). Most of the basin has been mapped only in a reconnaissance fashion and the stratigraphy and nomenclature of the Cenozoic units has recently undergone major refinement. Figure 3 is a geologic map of the East Pisco Basin compiled from Balarezo et al. (1980), preliminary geologic quad sheets from INGEMMET, and our own field work. Figure 4 shows the location of areas referred to in the text and sections measured by Rice/Duke/U. New Mexico geologists during 1985-1987. Latitudes, longitudes, and a key to section abbreviations are given in Table I. Only representative sections are presented in this paper; additional section descriptions are given in Dunbar and Baker (1988), Marty (1989), and Stock (1989). Pre-Tertiary plutonic, volcanic and metamorphic rocks occur throughout the basin, but they are concentrated along the central and southern coast. Basement topography clearly played a major role in this history of sedimentation in the basin; we have observed sedi-

239

ments of Late Eocene through Pliocene age in direct depositional contact with crystalline basement units within the Rio Ica Valley, Rio Grande Valley, Paracas Peninsula, and Quebrada Huaracangana. The geometry of the basement outcrops and mounting evidence for extensive subaerial exposure of basement highs suggests that during much of the Cenozoic, the Pisco Basin was comprised of 3 or 4 isolated or semi-isolated embayments. DeVries (1988a) points out that alternating periods of partially restricted circulation and relatively open exposure to the Pacific Ocean has produced a complex sequence of paleoenvironments that precludes simple basin-wide classification. At least 3 major transgressions inundated the East Pisco Basin during the Cenozoic, forming three sedimentary sequences that exhibit similar facies progressions (Fig.5). Fossiliferous conglomerates, sandstones, and oyster and barnacle bioherms of each transgressive event onlap basement highs. Basement relief is typically quite large; lateral and vertical facies changes preserved near such features are often spectacular. These coarsegrained units grade laterally and upwards into sandstones and siltstones; the upper portion of each transgressive unit is capped by a biogenic facies, either diatomites and diatomaceous siltstones and mudstones, or siltstones and mudstones with dolomitic intervals. The sedimentary rocks of the Pisco Basin are divisible into at least 7 mappable units, the Caballas Fm., Los Choros fm., Yumaque fm., Chilcatay fm., Pisco Fm., Cafiete Fm., and Changuillo Fm. Figure 6 (A-E) is a summary diagram of the stratigraphy and nomenclature suggested for the East Pisco Basin. Limited age control is provided by molluscan evidence (DeVries, 1987, 1988b), diatoms (Fourtanier and Machar~, 1988; Schrader and Ronning, 1988; Tsuchi et al., 1988a; R. Arends, pers. comm., 1989). radiolarians (Marty, 1989; and this paper), foraminifera (Tsuchi et al., 1988a; H. Heitman, pers. comm., 1989), nannofossils (R. Rosen and M. Filewicz, pers. comm., 1989),

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S~Sr/S6Sr ratios (P. Baker, pers. comm., 1989), and several K/Ar age dates (Table II). Radiolarian zonal assignments for specific samples in the Pisco Basin and summaries of the diatom zonal assignments of Schrader and Ronning (1988) and Fourtanier and Machar6 (1988) are shown in Fig.7; additional section specific age assignments are given in Table III. For the most part, the zonal information in Fig.7 and Table III represents precise age assignments;

however, in some cases a possible zonal range for poorly dated samples is given. Until recently, the Pisco Basin was thought to contain only two Cenozoic marine units, the Paracas Formation, a clastic near-shore unit of middle to late Eocene age, and the Pisco Formation, a diatomaceous, tuffaceous, and phosphatic deeper water unit of middle to late Miocene age (Petersen, 1954; Balarezo et al., 1980; Fig.6A). Fourtanier and Machar6 (1988;

242

R.B. DUNBAR ET AL.

TABLE I Location of measured sections and well samples, Pisco and Sechura Basins Section

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Cerro Redondo Cerro La Bruja Cerro Blanco Cuesta Chilcatay Cerro Fosfato Cerro Los Quesos Cerro Terrestral Cerros La Virgen Fundo Desbarrancado Fundo Santa Rosa Huanami Bridge Cer. La Yesera Central Cerro La Yesera South Monte Caucato Monte Caucato Montera Hacienda Molde de Queso Molde de Queso #2 Huaca Los Molinos Playa E1 Erizal #2 Punta E1 Puente Playa Palo Venta Quebrada Perdida Punta Prieto Cerro Lechuza Playa Yumaque Cerro Queso Grande Q. Huaricangana Q. Huaricangana Q. Huaricangana Q. Huaracangana Quebrada Santa Cruz Robyn's Chilcatay Quebrada Santa Cruz San Juan San Juan Yesero de Amara

14°22'S 14°32'S 14°24.5'S 14°16'S 14°55.6'S 14°30'S 14°51.5'S 14°10'S 14°49'S 14°47'S 13°41.5'S 14°32'S 14°34'S 13°38'S 13°38'S 13°40'S 14°56.2'S 14°56'S 13°42'S 14°7.5'S 13°59.5'S 14°22'S 14°33'S 13°54.5'S 13°57'S 14°55'S 14°26'S 14°58'S 14°58'S 14°57'S 14°57.2'S 14°54'S 14°57.2'S 15°55.5'S 15°20'S 15°22'S 14°35'S

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Fig.6B) and Machar~ et al. (1988a) refined the age of the Pisco Formation using diatoms and established the presence of a previously unknown uppermost Oligocene through lower

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CENOZOIC MARINE SEDIMENTATION IN SECHURA AND PISCO BASINS, PERU

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sediments of the Rio Grande Valley area and proposed elevating the Pisco For m at i on to Group status. In their classification scheme the lowermost Miocene unit is the diatomaceous Las Brujas formation. This is overlain by the Coyungo fm., a sandstone with interbedded diatomaceous siltstones, and the sequence is capped by the Te r r e s t r a l fm., comprised of approximately 150 m of reworked pyroclastic siltstones and sandstones. Davil~ et al. (1987) do not describe an Oligocene/Miocene unit in the Rio Grande Valley, but use the name ~'Caballas Fm." to designate a sequence of fluviatile sandstones which occur adjacent to P u e r t o Caballas ne a r the mouth of the Rio Grande. The precise age of these deposits has not been determined; but based on mapping in the area, Davil~t et al. (1987) consider the Caballas Fm. to underlie the upper Eocene

243

Paracas Formation. Along the coast in the n o r t h e r n and central Pisco Basin, the Paracas F o r m a t i o n has a finer-grained diatomaceous, organic-rich equivalent or successor which we have informally designated as "Yumaque form at i on" (Marty et al., 1988, Fig.6D). The national a u t h o r i t y in Peru for stratigraphic n o m e n c l a t u r e is the Instituto GeolSgico Minero y Metalilrgico (INGEMMET). In May 1988, I N G E M M E T geologists (including Davil~ and his colleagues) hosted a meeting on the development of a new stratigraphic framework for the Pisco Basin. The meeting was attended by T. DeVries (University of South Carolina) and J. Machar6 (Instituto Geofisico del Peril) as well as our group. A consensus was reached to elevate the Paracas to Group status. The coarse sediments previously assigned to the Paracas Form at i on would now be the Los Choros fm. and the finer-grained diatomaceous, porcelanitic, and organic mudstone units would be Yumaque formation. Because of their designation of pre-Paracas terrestrial sandstones on the coast near Puert o Caballas as Caballas Fm., I N G E M M E T is r e l u c t a n t to approve this name for the upper Oligocene to lower Miocene indurated siltstones described at several locations in the Pisco Basin by Machar6 et al. (1988). In coastal outcrops adjacent to the Bahia de la Independencia (Chilcatay Hills), T. DeVries and H. Schrader have also described a thick sequence of lowermost to middle Miocene tuffaceous siltstones and dolomites (DeVries, 1988c). Part of this same outcrop was sampled by Machar6 et al. (1988: Pampa Chilcatay) and assigned to the Caballas formation. In May, 1988, DeVries suggested the name Chilcatay fm. as an alternative designation for Oligocene to lower Miocene rocks of the Pisco Basin if the term "Caballas" remains intractable. We support this alternative designation; the name is unambiguous, the sequence at Pampa Chilcatay is representative of other sections of Oligocene/early Miocene sediments within the basin, and contacts with the overlying Pisco Formation are visible. On the issue of the division of the Pisco Fm.

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Fig.6. Composition of stratigraphy and nomenclature suggested for the Pisco Basin (A-E) and Sechura Basin (F). From Balarezo et al. (1980), Fourtanier and Machar~ (1988), Machar~ et al. (1988a), Dhvila et al. (1987), and Dunbar et al. (1988). Vertical lines indicate hiatuses. Wavy lines indicate unconformities; dashed lines indicate considerable age uncertainty. Lithologies of the major formations are described in text.

TABLE II K/Ar dates from Pisco Basin sections S a m p l e no. S e c t i o n

Location

Age (Ma)

QG-36 MC-2-9 QH-7/3-20 MQ-2-7

14°26'S75°46'W 13°38'S76°11'W 14°56'S75°16'W 14°56'S75°15'W

6.75__+0.18 6.89__+0.10 6.42_+0.28 5.49___0.12

Queso Grande Monte Caucato Huaracangana M. de Queso

s e n t s o u r o w n p r e f e r e n c e for n o m e n c l a t u r e a n d e s t i m a t e d a g e a s s i g n m e n t s a n d is n o t a n official p r o d u c t of t h e I N G E M M E T s t r a t i g r a p h i c colloquium. As f o r m a l t y p e s e c t i o n s a n d b o u n d a r y d e s c r i p t i o n s for t h e Los Choros, C h i l c a t a y , a n d Y u m a q u e f o r m a t i o n s h a v e n o t y e t b e e n approved, t h e s e d e s i g n a t i o n s r e m a i n informal. I n the following section, we briefly describe t h e p r i n c i p l e m a r i n e deposits of t h e P i s c o Basin.

Los Choros formation (Paracas Group) rocks into several new formations, there was a g e n e r a l g r o u p c o n s e n s u s to r e t a i n t h e P i s c o as a formation with subdivision into a lower s a n d y t u f f a c e o u s m e m b e r a n d a n u p p e r diatom a c e o u s unit. I N G E M M E T g e o l o g i s t s a r e c o n t i n u i n g w o r k in t h e Rio G r a n d e a r e a w h i c h m a y u l t i m a t e l y lead to a f o r m a l s u b d i v i s i o n of t h e P i s c o F o r m a t i o n in this a r e a . T h e s t r a t i g r a p h i c c o l u m n in Fig.6E repre-

T h e Los C h o r o s fm., p r e v i o u s l y d e s c r i b e d as P a r a c a s F o r m a t i o n by P e t e r s e n (1954), N e w e l l (1956), a n d B a l a r e z o et al. (1980), is c o m p r i s e d of up to 700 m of n e a r s h o r e a n d i n n e r s h e l f bioclastic conglomerates, sandstones, and lesser a m o u n t s of s i l t s t o n e a n d m u d r o c k . I n the n o r t h e r n P i s c o Basin, c o a r s e clastics of t h e r o c k y i n t e r t i d a l a n d s h o r e f a c e zones d o m i n a t e

245

CENOZOIC MARINE SEDIMENTATION IN SECHURA AND PISCO BASINS, PERU

PISCO BASIN

IRadiolariansl ~IDiatomsllSechuraBasl PiscoFro.(Northl[PlscoFm.(Central&South)1 I IRadiolarianl

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Fig.7. Radiolarian zonal assignments for the Pisco and Sechura Basins and compiled diatom zones recognized in samples from the Pisco Basin. Columns 1-25 show radiolarian zonal age assignments for specific samples from sections within the Pisco Basin (locations shown in Fig.4). Radiolarian checklists and photographs appear in Marty (1989). Column 26 is a composite of diatom zonal assignments for 12 samples from the Pisco Basin by *Fourtanier and Machar~ (1988). Fourtanier and Machar~ (1988) did not make a late Eocene diatom zone assignment, but list a number of diatom species characteristic of the late Eocene/early Oligocene. Column 27 is a composite of possible diatom zones recognized in 15 samples from the Pisco Basin by **Schrader and Ronning (1988). Columns 28 through 34 show radiolarian zonal ages for outcrop and well core samples from the Sechura Basin. Sample locations are given in Table I; radiolarian checklists are in Marty (1989). For both diatom and radiolarian biostratigraphy some zonal assignments reflect possible age ranges for poorly dated samples. Neogene time scale and zonal age assignments for diatoms and radiolarians is from Barron et al. (1985). Paleogene radiolarian zonation is after Riedel and Sanfilippo (1978) as reported in Berggren et al. (1985). Paleogene diatom zonation is from Gombos and Cielsielski (1983) and Fenner (1985) as summarized in Haq et al. (1987). Additional age assignments for Pisco and Sechura Basin samples are given in Table III.

the base of the formation, while the middle and upper portions consist of numerous coarsening-upward shelf-shoreface cycles (Fig.8A; Wright and Cruzado, 1988). An overall trend

toward increasing water depth upward in the formation is suggested by progressively deeper facies present in the shelf-shoreface cycles. The unconformity between the Paracas group

246 TABLE III Additional age estimates for Pisco and Sechura Basin sediments* Pisco Basin 1. MOLLUSCS QH87A1, Late Eocene Oligocene (Aturia alabamensis peruviana, B. Kues, pers. comm.); QH87A, Late EoceneOligocene (Turritella gilbertharrisi, T. DeVries, pers. comm.). 2. NANNOFOSSILS QH87A, Late Eocene (Isthmolithus recurvus, Discoaster barbadiensis, D. saipanensis), SC-88-35, Late Eocene, (Dictyococcites bisectus, Ericsonia formosa, and Reticulofenestra samodurovaj; R. Rosen, pers. comm.); PP-63 Zone Cnlc (Discoaster druggi) of Okada and Bukry (1980), Early Miocene (M. Filewicz, pers. comm.). 3. FORAMINIFERA RC-6, Zone N8 (Praeorbulina glomerosa), Latest Early Earliest Middle Miocene, PP-42, Miocene (Uvigerina carapitana, Lenticulina sp. Bathysiphon sp.), PY-21, Zone P12-P17 (Globigerinatheka index), Late Middle-Late Eocene (H. Heitman, pers. comm.); Pc-17-4, Zone N6-N7

(Globorotalia scitula praescitula, Globoquadrina dehiscens), Late Early Miocene, Pe-18, Zone P13 (Globigerinatheka index), Late Eocene (Tsuchi et al., 1988a). 4. DIATOMS RC-34, Zone Cestodiscus peplum, Early Middle Miocene (R. Arends, pers. comm.); Huanami Bridge, Diatom zones Nitzschia miocenica, N. jousea, and Pseudoeunotia doliolus, Late Miocene through Pleistocene, Ocucaje area, Diatom zones N. miocenica and N. fossilis Late Miocene (Tsuchi et al., 1988a). 5. s~SR/S6SR RATIOS in authigenic dolomites QSC-30, 0.707690, Late Middle-Early Late Eocene; PY-1, 0.707727, Late Eocene; PE-8, 0.707738, Late Eocene; PP-8, 0.707804, Late E9cene; pp.60, 0.708387, Latest OligoceneEarly Miocene (P. Baker, pers. comm.). Sechura Basin 1. DIATOMS (Tsuchi et al., 1988a) Punta Tric-Trac, Late Eocene (Melosira architecturalis, Pyrgypsis gracilis, and Triceratium barbaense); La Mina (MineroPeru), Late Miocene (Nitzschia cylindrica, N. fossilis, and N. porteri); Pampa Los Hornillos, Late MiddleLate Miocene (Coscinodiscus plicatus, N.porteri, and

Rossiella paleocea). *Location of all samples except those referenced to Tsuchi et al. (1988a) is given in Table I and Fig.4.

and pre-Tertiary crystalline and sedimentary basement is one of the most spectacular in the Pisco Basin with local relief up to 30 m. The upper boundary of the Los Choros fm. appears

R. B. DUNBARET AL.

to be gradational into the upper Eocene Yumaque formation. Age of the Los Choros fm. in the Paracas Peninsula area is well constrained as mid to late Eocene based on abundant macrofossil data (Lisson, 1925; Rivera, 1957), foraminiferal age assignments (Zone P13; Tsuchi et al., 1988a) and stratigraphic relationship to upper Eocene diatomaceous deposits (Marty et al., 1988; Fourtanier and Machar~, 1988). The oldest strata to onlap the crystalline basement in the southern Pisco Basin (Quebrada Huaracangana, Fig.4) consist of lobate breccia conglomerates that are vertically gradational into coarse fossiliferous marine sandstones. Maximum age is well-constrained as late Eocene by numerous specimens of the nautiloid Aturia alabamensis (B. Kues, pers. comm., 1988) and the gastropod Turritella gilbertharrisi (T. DeVries, pers. comm., 1989) within the upper marine sandstones. These strata preserve the transition from coastal fandelta to marine shelf conditions (Wright and Cruzado, 1988; Stock, 1989) during initial basin submergence, and are assigned to the Los Choros formation.

Yumaque formation (Paracas Group) The Los Choros formation sandstones and shell hash layers are overlain by fine-grained marine mudrocks, phosphatic shales, diatomites, porcelanites, and cherts of the Yumaque formation. The contact with the Los Choros fm. is not well defined, but appears to be gradational. In Yumaque sections we have examined to date, the transition from coarse to fine-grained deposition occurred during the Middle to Late Eocene Calocyclas azyx radiolarian zone (Fig.7). The most diatomaceous units were deposited during the late Eocene C. bandyca zone and are best exposed at Fundo Desbarrancado (Fig.4; Marty et al., 1988). The section at Fundo Desbarrancado consists of about 40 m of laminated and massive diatomite with nodular chert horizons (Fig.8B). The regional extent of the Yumaque formation is presently unknown, however, upper Eocene

""

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~ig.8. Summary lithologies of measured sections representative of the Los Choros fm. (A), Yumaque fm. (B, C), and Chilcatay fm. (upper 250 m of D). Locations given n Fig.4. Age dates of samples from Fundo Desbarrancado, Playa el Erizal, and Quebrada Huaracangana (RC) are given in Fig.7 and Table III. Section A provided

*UNTA PRIETO (PRI)

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=

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>

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248

fine-grained biogenic sediments crop out over large areas of the northern and central coastal areas of the basin. Fourtanier and Machar6 (1988) report latest Eocene/earliest Oligocene diatom assemblages from Paracas Formation outcrops near Salinas de Otuma and Punta Lomitas. Late Eocene radiolarians occur at Quebrada Perdida, Quebrada Santa Cruz, and Playa Yumaque (Sections PQ, QSC, and PY; Figs.4 and 7). Based on recently completed Sr isotope analyses of dolomite concretions (P. Baker, pers. comm., 1989), the coastal sections at Playa E1 Erizal and Punta E1 Puente are roughly the same age as the late Eocene siliceous rocks exposed at Playa Yumaque (Table III). The section at Playa E1 Erizal (Fig.8C) consists of nearly 200 m of calcareous porcelanite and porcelaneous mudstone with locally abundant phosphate nodules and fish debris. Organic carbon contents of outcrop samples from the upper laminated porcelaneous mudstones average about 1.5%, much higher than in the unlaminated sediments at the base of the section (0.2%). Aggregate stratigraphic thickness of the Yumaque fm. is greater than 150 m.

Chilcatay formation Machar6 et al. (1988a) originally used the name Caballas formation for recently discovered sediments of late Oligocene to early middle Miocene age. The name choice was based on the belief that a thick section of rocks of this age occur within Quebrada Santa Cruz (section QSC, Fig.4), adjacent to the coastal village of Puerto Caballas. However, based on radiolarian studies (Fig.7), Sr isotope data and nannofossil analysis (R. Rosen, pers. comm., 1989), most of the Quebrada Santa Cruz section appears to be late Eocene to early Oligocene in age (Table III). To our knowledge, the best exposed sections of latest Oligocene to early middle Miocene rocks within the Pisco Basin occur at Pampa Chilcatay, approximately 100 km NW of Puerto Caballas. Given the reluctance of I N G E M M E T to accept the Machar6 et al. (1988a) nomenclature, we have

R.B. DUNBAR ET AL.

adopted the name Chilcatay formation for these rocks. The Chilcatay fm. consists of indurated siltstones and sandstones with interbedded diatomaceous intervals. Approximately 200 m of this unit is exposed at Pampa Chilcatay (Fig.4). Using diatom stratigraphy for age control (Schrader and Ronning, 1988), DeVries (1988c) defines three transgressive cycles within the Chilcatay formation, beginning at approximately 24 Ma (Rocella gelida zone of Barron, 1985), 20 Ma (Craspedodiscus elegans zone), and 18.7 Ma (Triceratium pileus zone). The deeper water portions of each sequence consist of dolomite-cemented tuffaceous siltstones. Many of the diatoms recovered from the Chilcatay sections have a tropical and equatorial Pacific affinity (Fourtanier and Machar6, 1988). Upper Oligocene-Lower Middle Miocene biogenic sediments have also been recovered from the San Juan/San Nicholas region and Quebrada Santa Cruz (Fig.7 and DeVries, pers. comm., 1987). Machar6 and Fourtanier (1987) estimate the stratigraphic thickness of the Chilcatay formation at about 250 m. The Chilcatay formation in Quebrada Huaracangana (southern Pisco Basin, Fig.4) is comprised of at least two transgressive sequences. These units include siltstones and interbedded dolomites grading upwards into sandier facies which are in turn overlain by finer-grained biogenically enriched deposits (Fig.8D). Dolomites commonly occur as large (0.5-2 m) nodules in the finer-grained intervals. Siltstones are often laminated and may contain phosphate nodules. The upper part of the Chilcatay formation in the Huarancangana area has been assigned to the N8 (Praeorbulina glomerosa) foram zone (H. L. Heitman, pers. comm., 1989) and Cestodiscus peplum A diatom zone (R. G. Arends, pers. comm., 1989), both of earliest middle Miocene age (approximately 16.4-15 Ma).

Pisco formation The Chilcatay formation is unconformably overlain by the upper middle Miocene through

CENOZOICMARINESEDIMENTATIONIN SECHURAANDPISCOBASINS.PERU P l i o c e n e Pisco F o r m a t i o n . The Pisco Formation has been studied by m a n y p r e v i o u s workers i n c l u d i n g A d a m s (1908), P e t e r s e n (1954), R u e g g (1956), M e r t z (1966), a n d B a l a r e z o et al. (1980). The Pisco Fm. consists of a wide v a r i e t y of lithologies i n c l u d i n g b i o c l a s t i c conglomerates and s a n d s t o n e s n e a r b a s e m e n t h i g h s (Fig.9A), t u f f a c e o u s siltstones, sandstones, and

249

ash horizons, d i a t o m a c e o u s siltstones, diatomites, p h o s p h o r i t e s , dolomites, and m i n o r limestone. The t h i c k e s t Pisco u n i t is a t u f f a c e o u s member, t y p i c a l l y f o u n d n e a r the base of sections a l o n g the Rio G r a n d e and Rio Ica valleys in the c e n t r a l a n d s o u t h e r n p a r t of the b a s i n (Figs.10A and 13). A l t h o u g h gravel and s a n d s t o n e l a y e r s are i n t e r s p e r s e d t h r o u g h o u t

Fig.9. A. Upper Miocene bioclastic pebble conglomerates and sandstones onlapping volcanic basement in the Rio Ica Valley. In the background is a thick section of tuffaceous siltstone (Cerro Blanco). B. Phosphate pebble conglomerate bed in the Pisco Formation at Quebrada Huaracangana. 5 cm lens cap at lower right for scale.

iO

0

50-

(CB)

3ERRO BLANCO

3

3eds & .3hannels ~f Phosphate ~ong Iomerate

uffaceous ;andstone,

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_=

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(CF)

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u

Siltstone

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(QG

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/ |

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/

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/

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} U E S O GRANDE

~hell M a t e r i a l

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Vludstone

rut faceous

rut faceous Diatomite

3iatomite

~andy Siltstone

e

~ig.10. Summary lithologies of measured sections of the Pisco Formation. Locations given in Fig.4. K/Ar dates for the Molde de Queso and Queso Grande sections r e in Table II; radiolarian dates for Molde de Queso are in Fig.7.

(CT)

CERRO "ERRESTRAL

/IOLD E de QUESO (MQ)

.'ERRO

3

Sandstone

o

CENOZOIC MARINE SEDIMENTATION IN SECHURA AND PISCO BASINS, PERU

the section, the dominant lithology is tuffaceous sandy siltstone. These sediments display abundant evidence of current and wave reworking, including large and fine-scale cross-bedding. Large soft-sediment deformation structures attest to slumping and rapid dewatering in some intervals. The middle section of the Pisco Fm. contains regularly spaced dolomite cemented beds or nodule horizons (Fig.10B). The highly variable carbon isotopic composition of these dolomites supports an origin related to early diagenesis of organic matter (Martin, 1987). Phosphatic sediments in the Pisco Formation occur in a wide variety of forms including pellets, ooids, indurated spherical nodules (1 5 cm), nodule conglomerates (Fig.9B), coatings on shells, and hardgrounds (Allen et al., 1987). In Quebrada Huaracangana, up to 23 regularly spaced phosphate pebble conglomerate beds formed within a 300m section deposited between approximately 6.4 and 5.5 Ma (TableII; Fig.10C, D). We interpret these phosphate pebble horizons as lags produced by cyclical current or wave-induced winnowing events, possible linked to high frequency sealevel changes during the Messinian desiccation events. In the central and southern parts of the Pisco Basin, most sections become more diatomaceous near the top of the Pisco Formation (Figs.10C, E); opal contents typically range from 25 to 50 weight °/o. The thickest section of diatomaceous sediment occurs at the northern limit of the onshore portion of the basin along the Rio Pisco (sections MC, HB, and OLPH; Fig.4). Here, approximately 500 m of diatomite and diatomaceous mudstone, averaging > 30% opal by weight, has accumulated between the latest middle Miocene and early Pliocene. The onset of significant biosiliceous sedimentation is time transgressive in the Pisco Basin, beginning about 11 12 Ma in the north, 6 7 Ma in the central basin, and 4 5.5 Ma in the south (see Fig.13). Radiolarian (Fig.7 and Marty, 1989), diatom (Schrader and Ronning, 1988; Fourtanier and Machar~, 1988; Tsuchi et al., 1988a), and

251

foraminiferal (Tsuchi et al., 1988a) data document the age of the Pisco Formation as Late Middle Miocene (10 12 Ma; Diartuspetterssoni radiolarian zone, Actinocyclus moronensis diatom zone) through middle Pliocene to early Pleistocene (Spongaster pentas radiolarian zone, Nitzschia reinholdii diatom zone). Figure 7 and data from Schrader (pers. comm., 1986) suggests t h a t many of the thick Pisco sections along the Rio Ica Valley were deposited over a more limited time period, from 7 to 5 Ma. K/At dates for the Pisco Formation (Table II) range from 6.9 to 5.5 Ma and bracket the interval of greatest phosphate deposition in the Huaracangana area between 6.4 and 5.5 Ma. The stratigraphic thickness of the Pisco Formation varies greatly throughout the onshore portion of the basin, ranging from about 200 to 1000 m.

Sechura Basin stratigraphy The Sechura Basin of northern Peru is bounded on the west by the Precambrian and Paleozoic rocks of the Cerros Illescas, an onshore extension of the Outer Shelf High, and to the east by the Andean Batholith (Fig.l). The boundary between the onshore ( ~ 20,000 km 2) and offshore ( ~ 10,000 km 2) portions of the basin roughly coincides with the seaward extension of the Huancabamba Deflection. Late Cenozoic uplift within the Sechura Basin has been relatively minor; much of the onshore portion of the basin is at or slightly below sealevel. Information on Cenozoic sediment fill is derived from limited outcrops along the coast and central depression and several wells drilled for petroleum and phosphate mineral exploration (Travis et al., 1976; Cheney et al., 1979; Caldas et al., 1980; Ochoa, 1980). Sedimentary units range from late Eocene to Pleistocene in age; total thickness ranges up to 1500-2300 metres. The oldest Cenozoic sedimentary sequence in the Sechura Basin is late Eocene in age and consists of the Verdun Formation, a calcareous sandstone and shale unit, overlain by the Chira Formation, composed of calcareous sandstones and diatomaceous shales (Figs.6F

252

and 11). In 1984, we sampled Chira Formation outcrops in the sea cliffs at Bayovar. As described by Caldas et al. (1980), the Chira Formation bears a striking similarity to the upper Eocene diatomaceous shales of the Yumaque formation in the Pisco Basin. However, the opal in these coastal outcrops (5-10% by weight) is made up exclusively of radiolarians; diatom tests appear to have been removed by dissolution. We have assigned the radiolarian fauna present in these outcrops (PPQ-1) to either the Calocyclas azyx or lower part of the C. bandyca radiolarian zone (Fig.7), equivalent in age to Yumaque fm. diatomites at Fundo Desbarrancado in the Pisco Basin. Calcareous nannofossils and diatoms from the Bayovar outcrop have also been assigned to the Late Eocene (NP17-20, Tsuchi et al., 1988a; and Table III). Estimated thickness of the Verdun/Chira Fms. ranges from 300 to 900 m (Caldas et al., 1980). The Verdun/Chira Formations are unconformably overlain by the lower Miocene Mancora and Heath Formations. These units do not crop out in the Sechura Basin but are known from well samples and have been correlated to outcrops and additional well samples in the Talara and Tumbes Basins (Travis et al., 1976; Caldas et al., 1980; Ochoa, 1980). The sequence consists of approximately 300 m of upwardsfining terrigenous clastics. Although ages of undifferentiated Oligocene were originally assigned to the Heath Formation based on benthic foraminifera, a reevaluation of foraminiferal assemblages at the type section of the Heath Fro. by Cruzado and Sanz (1973) yields only Miocene ages. Our own radiolarian studies of Mancora/Heath samples from a well core in the Sechura Basin (VIRU 4X1 1400) yield an early Miocene age (Stichocorys delmontensis through S. wolffii radiolarian zones; Fig.7). These units are thus time-equivalent with the Chilcatay fm. of the Pisco Basin. The Mancora/Heath Formations are unconformably overlain by the interbedded sandstones and bioclastic limestones of the Montera Formation. These coarser nearshore sediments grade upwards into the finer-grained

R.B. DUNBAR ET AL.

~ -iI Sand =r "o u ) -~. . 1 1 o 0>

Diatomite & Phosphorite

¢

Diatomite & Phosphorite

-

N

~,~' o~

Diatomite & Tuff

Mudstone ro

. . . . . . . . . .

-..--:....--..-_

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o

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i 200

:--;j.....

~1 -~:- -=---:.~

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400

- _--,~.,- _-,._~_ _-~.

m

Diatomaceous Shale

Dolomitic Sandstone Tuffaceous Sandstone Calcareous Sandstone

ff~'~.::"" Fissile Shale

Fig.ll. Composite stratigraphic column for Cenozoic sediments of the Sechura Basin. After Cheneyet al. (1979) and Caldas et al. (1980).

CENOZOIC MARINE SEDIMENTATION IN SECHURA AND PISCO BASINS, PERU

facies of the lower Zapallal Formation. The Zapallal Formation consists of up to 1200 m of sandy mudstone grading upwards into mudstone, diatomaceous mudstone, and diatomite (Cheney et al., 1979; Caldas et al., 1980). Radiolarians recovered from the Zapallal Formation span the Dorcadospyris alata to Stichocorys peregrina radiolarian zones (Fig.7). Based on sedimentation rates extrapolated from radiolarian age estimates, Marty (1989) estimates the age of the base of the Zapallal Formation at 14+ 1.5 Ma. The lower and upper members of the Zapallal Fm. are separated by an unconformity marked by a dolomite-cemented sandstone hardground. The sediments below this unconformity are gently folded and truncated while those above are flat-lying. We have previously suggested that this feature resulted from uplift at approximately 7 8 Ma in response to subduction of the aseismic Nazca Ridge (Marty et al., 1987). Biogenic sediments occur in both the lower and upper Zapallal Formation. Immediately below the late Miocene unconformity are a series of phosphate ore horizons, described by Cheney et al. (1979). The phosphate occurs mainly as 1 - 2 m thick beds of well-sorted phosphate peloids, many of which are phosphatized foraminifera. Opal contents of the interbedded diatomites may exceed 70% by weight. Immediately above the unconformity lies a discontinuous phosphorite bed which is overlain by up to 75 m of diatomite. The interval of diatomaceous and phosphatic sedimentation within the Zapallal Formation occurred between about 8.5 and 4.5 Ma (Didymocyrtis antepenultima through Stichocorys peregrina radiolarian zones). The upper Zapallal Fm. is unconformably overlain by the Pliocene/Pleistocene continental deposits of the Miramar and Hornillos formations. B i o g e n i c s e d i m e n t a t i o n in the P i s c o and S e c h u r a basins The modern Peruvian upwelling system is one of the most highly productive areas of the ocean, with long-term surface water carbon

253

fixation rates exceeding 1 g Corg m-e day 1 (Zuta and Guill6n, 1970). The high rate of supply of organic matter to the seafloor contributes to the development of a strong oxygen minimum zone on the outer shelf and upper slope and greatly influences authigenic mineral formation. Upwelling associated deposits include diatomites, organic-rich shales, secondary carbonates, and phosphorites (Veeh et al., 1973; Burnett, 1977; Froelich et al., 1988; Glenn and Arthur, 1988; also see Bentor, 1980). Similar associations are observed in both Neogene and Paleogene sedimentary sequences along the Peruvian margin, implying that high fertility in surface waters and low oxygen levels at the seafloor have been a recurrent feature of the coastal ocean during the last 40 m.y. Biogenic sediments were deposited in the Pisco Basin during each of three major transgressive events of the Tertiary. Organic dolomites are common in the deeper water portions of all three sequences. Phosphatic sediments are found in sections with both shallow and deeper water characteristics and are often associated with secondary carbonates. Diatomaceous sediments were likewise deposited during each transgression, although, only as discontinuous diatom beds in the Chilcatay fm.; thick diatomites are only found in the Yumaque and Pisco formations. Water depth and/or protection from detrital input are important controls on the occurrence of diatomaceous facies, however, we note that many biogenic units in the Pisco Basin exhibit shallow water sedimentary structures or occur very close to nearshore deposits. Based on the preliminary biostratigraphies of Fourtanier and Machar6 (1988), Marty (1989), and Schrader and Ronning (1988), episodes of diatomaceous sedimentation occurred from about 40 to 36 Ma, 24 to 16 Ma, 11 to 3 Ma. Sechura Basin sediments were deposited during 4 major transgressive events, however, diatomaceous sedimentation was confined to the periods of roughly 40-37 Ma and 8.5-4.5 Ma. In both the Pisco and Sechura Basins, opalenriched sediments were deposited during the

254

late Eocene as well as late Miocene suggesting that coastal upwelling was not restricted to the Neogene. In most of the Pisco Formation sections we have examined to date, opal content increases upsection into the uppermost Miocene-lower Pliocene interval (e.g. Fig.10E) possibly reflecting the time of greatest transgression as well as a late Miocene intensification of the upwelling system. Intensified coastal upwelling acts in two ways to enhance diatomaceous sedimentation; greater nutrient supply increases primary production, and the presence of cold water increases coastal aridity, reducing the production and input of finegrained terrigenous phases. Late Miocene radiolarian assemblages in the Pisco Basin are quite different from Eocene faunas and document much cooler waters within t h e Neogene upwelling system. Fourtanier and Machar~ (1988) and Schrader and Ronning (1988) report equatorial and tropical diatom assemblages from the lower Miocene sections of the Chilcatay fm. and a noticeable increase in cold water forms in the upper Miocene Pisco Formation. In summary, although coastal upwelling has very likely occurred along the Peruvian margin since at least the late Eocene, strengthening and cooling of the upwelling system occurred during the late Miocene. Late Neogene marine sedimentation in the basins was terminated by episodes of uplift, during the early Pliocene in the Sechura Basin, and late Pliocene/Pleistocene in the Pisco Basin, quite possibly linked to the passage of the subducting Nazca Ridge. Biogenic sediments have continued to accumulate throughout the Quaternary in the northern, offshore portion of the Pisco Basin where ODP Leg 112 recovered over 300 m of Pleistocene through Recent organic-rich diatomaceous muds (Site 686, Von Huene et al., 1987). Upper Tertiary sediments from the Peruvian margin are similar to those from northern circum-Pacific basins described by Ingle (1973, 1981) and Pisciotto and Garrison (1981), including the Onnagawa and F u n a k a w a Fms. of Japan, the Sisquoc and Monterey Fms. of California, and the Tortugas Formation of

R.B. DUNBAR ET AL.

Mexico. Less well-known Neogene biogenic deposits on the west coast of Latin America which are roughly time-equivalent include the Borbon, Progreso, and Villingota Fms. of Ecuador (Ortega et al., 1982; Hasson and Fischer, 1986), the Mejillones and Tongoy deposits of Chile (Martinez-Pardo and Caro, 1980; Tsuchi et al., 1988c), and the Zapallal, Chilcatay, and Pisco Fms. of Peru. Correlations between biosiliceous deposits at 7 circumPacific locations are shown in Fig.12. Although the basal age of diatom-bearing Late Neogene marine formations in Ecuador, Peru, and Chile is roughly synchronous at 12-15 Ma (within the limits of present age control), the most biosiliceous intervals of each formation occur at different times. The onset of highly diatomaceous sedimentation ranges from 5 to 12 Ma; biogenically enriched intervals span periods ranging in length from 3 to 8m.y. Although the precise timing of onset and cessation of biosiliceous sedimentation at specific locations in the Latin American forearc is controlled to a large extent by local tectonism, there does appear to be a global climatic or oceanographic link. With the exception of the Mejillones deposits of Chile, the onset of biosiliceous sedimentation within the Latin American regions illustrated in Fig.12 occurs between approximately 8 and 12 Ma, the same time interval during which diatomaceous sediment began to accumulate in J a p a n and California. Observations from these Latin American deposits support Ingle's (1981) suggestion that the mid-Miocene increase in biogenic sedimentation in the North Pacific resulted from a global intensification of atmospheric and oceanic circulation. Paleogene biogenic deposits on the Peru margin can be correlated to other biosiliceous units in Ecuador and California (Fig.12) and imply that episodic coastal upwelling within eastern boundary cUrrents may have occurred since at least the Late Eocene. If episodes of strong coastal upwelling reflect Southern Ocean cooling and strengthening of the Peru Current, these observations support the hypothesis that significant Antarctic cooling

CENOZOIC MARINE SEDIMENTATION IN SECHURA AND PISCO BASINS, PERU

Ma

0

RADIOLARIAN ZONES

ECUADOR BORBON BASIN

PERU SECHURA BASIN

255

CH,LE 1 CH'LE I'CAL'FORN' I 'APAN (NORTHERN (WEST

PERU PISCO BASIN

23 ° S

30°8

&CENTRAL

COASTI

L. H a y s i P. prismatium S. pentas

C"'taI

S. p e r e g r i n a D. penultima -~

10

antepenultima

D. oetterssoni

Borbon Fm. Zapallal I ' Fm. Pisco Fm.

D

Herradura de Mejillones

? Diatomita deTongoy

alata

15

I IOnna0awa Fm.

Monterey Fm.

C, costata woIfii

20

S. d e l m o n t e n s i $

Chilcatay fm.

C. tetrapera L, e l o n g a t a

ONZOLE/ TOSAGUA

25 O. ateuchus

30

tuberosa

35 C. ornata C. bandyca

40

C. a z y x

San Mateo Sh.

Chira

P, chalara

Kreyenhagen

Fig.12. Ages of Cenozoic units known to contain biosiliceous sediments in selected regions of Ecuador, Peru, Chile, California, and Japan. Thick vertical lines within formation boundaries represent intervals of greatest enrichment in biogenic silica, where known. Dashed lines or question marks indicate considerable age uncertainty. Sources for Ecuador: Borbon Fm. diatomites from Hasson and Fisher (1986), Onzole/Tosagua Formations from Stainforth (1948) and Tsuchi et al. (1988b), "San Mateo Shale" from Stainforth (1948). Biosiliceous nature of the Onzole Fm. and San Mateo Shale from Stainforth's description of an enriched, deeper water "radiolarian facies" with a cool water aspect; the diatom content of these rocks is unknown. Sources for Peru: this paper (note that diatomaceous sediments occur in the Chilcatay fm. only as infrequent interbeds). Sources for Chile: at 23°C, Tsuchi et al. (1988c), at 30°S, Martinez-Pardo and Caro P. (1980; foram zones N13 15). Sources for California: Ingle (1981) and Blueford (1984). Source for Japan: Iijima et al. (1988). Time scale and zonal assignments as in Fig.7.

b e g a n b y t h e l a t e E o c e n e (e.g. B a r r o n e t al., 1988; P r e n t i c e a n d M a t t h e w s , 1988), w e l l b e f o r e the important mid-Miocene event described by S a v i n e t al. (1975), K e n n e t t (1977), a n d m a n y others. F i g u r e 13 i l l u s t r a t e s h i g h l y g e n e r a l i z e d Neogene stratigraphic progressions for the M o n t e r e y / S i s q u o c F o r m a t i o n s of California, the Sechura Basin, the northern, central, and

s o u t h e r n p o r t i o n s o f t h e P i s c o B a s i n , a n d for s e d i m e n t c o r e s r e c o v e r e d d u r i n g O D P L e g 112. Neogene siliceous sediments are common in Californian and Peruvian marginal basins and are generally more abundant in the upper Miocene-Pliocene i n t e r v a l . P h o s p h a t i c dep o s i t s of o n s h o r e P e r u a r e r e s t r i c t e d to t h e latest Miocene, somewhat younger than the p h o s p h a t i c facies of the M o n t e r e y F o r m a t i o n .

256

R . B . D U N B A R ET AL.

California RADIOLARIAN ZONES

ia

0

J L

(Monterey, Sisquoc Fms. et al.)

L. Haysi

pcntas

Northern

Central

Southern

ODP Leg 112

Cont. elastics

P. prismatium

~_,,,S.

Pisco Basin Sechura Basin

-~.

CLASTICS

Miramar & Hornillos Fins. !

• S. oeregrina

I uJ D. penultirna ~_j D. antepenultima

10

_

D. ~etterssoni

.! ?

UJ _J Q j j E3 D. a/ata

15

20

JJ

~I?

.~-

~i

a

I

Mudstones

I1.

'~--

C. c o s t a t a

~;

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w

I

J" =o_. ou~l l-o @ E - .o o ? ~.

T:-,,

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= ~=

siltstones and sandstones

°I

25

Fig.13. Comparison of Neogene sedimentary facies occurrences in the Monterey Fm. and associated units, California (Pisciotto and Garrison, 1981, with time scale adjusted according to Berggren et al., 1985), the Sechura Basin of northern Peru (Cheney et al., 1979; Caldas et al., 1980; Marty, 1989; this paper, and K/Ar data, unpublished), the northern, central, and southern portions of the Pisco Basin (this paper), and drill cores recovered during ODP Leg 112 (Von Huene et al., 1987). Shading represents major hiatuses within the Sechura and Pisco Basins. Thickness of vertical lines roughly represents the relative abundance of different sedimentary components. Radiolarian zonal ages from Barron et al. (1985). A u t h i g e n i c c a r b o n a t e s , m a i n l y dolomite, a r e also c o m m o n c o m p o n e n t s of N e o g e n e b i o g e n i c s e d i m e n t s in b o t h P e r u a n d C a l i f o r n i a . T h e p r e c i s e t i m i n g of '
in t h e c e n t r a l a n d s o u t h e r n P i s c o Basin• In O D P L e g 112 cores, t h e oldest p h o s p h o r i t e s r e c o v e r e d a r e m i d - M i o c e n e ( ~ 13 Ma) a n d the m o s t d i a t o m a c e o u s a n d p h o s p h o r i t e - r i c h sedim e n t s a p p e a r r e s t r i c t e d to the u p p e r m o s t M i o c e n e t h r o u g h R e c e n t (Von H u e n e et al., 1987)• T h i s d i s t r i b u t i o n m a y reflect a s a m p l i n g bias in t h a t r e l a t i v e l y few cores older t h a n midM i o c e n e w e r e recovered• N o n e t h e l e s s , t h e o b s e r v e d d i a c h r o n e i t y of b i o g e n i c s e d i m e n t a r y u n i t s in the P e r u v i a n N e o g e n e s e c t i o n s suggests t h a t local s u b s i d e n c e / u p l i f t h i s t o r i e s p l a y e d a m a j o r role in c o n t r o l l i n g t h e occurr e n c e of t h e s e facies w i t h i n e a c h basin. T h e influence of N a z c a Ridge s u b d u c t i o n on biog e n i c s e d i m e n t a t i o n is p a r t i c u l a r l y w o r t h e x a m i n a t i o n , as t h e ridge h a s t r a v e r s e d n e a r l y the e n t i r e P e r u v i a n f o r e a r c f r o m n o r t h to s o u t h d u r i n g t h e l a t e N e o g e n e period of m a j o r c l i m a t i c a n d o c e a n o g r a p h i c change• A c o n s e r v a t i v e e s t i m a t e for the v o l u m e of Neogene sediment deposited within the central P e r u v i a n f o r e a r c is 175,000km 3 (Table IV).

CENOZOIC MARINE SEDIMENTATION IN SECHURA AND PISCO BASINS, PERU

257

TABLE IV Volume of Neogene sediments in Central Peru forearc basins Basin

Area (103 km 2)

Neogene sediment thickness* (m)

Neogene sediment volume (103 km 3)

Sechura Salaverry Pisco Yaquina Lima Trujillo

29 54 25 20 45 20

1000 500-1000 1000 9 1000 1300

29 40 25 107 45 26

Total

193

930 (ave.)

175

*Neogene sediment thickness from Travis et al. (1976), Schrader and Cruzado (1988), Thornburg and Kulm (1981), Kulm et al. (1982), Caldas et al. (1980).

T h i s is n e a r l y t w i c e t h e e s t i m a t e d v o l u m e of the M o n t e r e y Fm. in o n s h o r e a n d offshore C a l i f o r n i a ( P i s c i o t t o a n d G a r r i s o n , 1981). I f s i m i l a r styles of s e d i m e n t a t i o n w e r e m a i n t a i n e d t h r o u g h o u t the forearc, t h e P e r u m a r gin was a n i m p o r t a n t N e o g e n e c a r b o n s i n k a n d m a y h a v e c o n t r i b u t e d to p o s t u l a t e d c h a n g e s in a t m o s p h e r i c CO 2 levels. V i n c e n t a n d B e r g e r (1985) h a v e s u g g e s t e d t h a t c a r b o n a c c u m u l a t i o n in the M o n t e r e y F o r m a t i o n a n d o t h e r N o r t h Pacific e q u i v a l e n t s led to r e d u c e d p C O 2 levels d u r i n g the middle M i o c e n e , a n e v e n t t h e y h a v e l i n k e d to a m a j o r i n c r e a s e in A n t a r c t i c ice volume. O r g a n i c r i c h s e d i m e n t s of t h e Pisco a n d S e c h u r a B a s i n s w e r e d e p o s i t e d a f t e r this event, b u t m a y h a v e c o n t r i b u t e d to late M i o c e n e / P l i o c e n e c a r b o n f l u c t u a t i o n s . T h e t i m i n g a n d m a g n i t u d e of c a r b o n flux to t h e r e m a i n d e r of the f o r e a r c is n o t y e t k n o w n .

Summary Cenozoic s e d i m e n t s of the E a s t P i s c o B a s i n w e r e d e p o s i t e d d u r i n g at l e a s t 3 m a j o r t r a n s gressiwe cycles. T h e oldest s e q u e n c e is mid to l a t e E o c e n e a n d includes the n e a r s h o r e / i n n e r s h e l f s a n d s a n d c o n g l o m e r a t e s of t h e Los C h o r o s f o r m a t i o n ( P a r a c a s G r o u p ) w h i c h unconformably overlie a high relief pre-Tertiary b a s e m e n t surface. T h e Los C h o r o s f o r m a t i o n g r a d e s u p w a r d s into d i a t o m a c e o u s a n d porce-

lanitic s h a l e s a n d d i a t o m i t e s of t h e u p p e r m o s t Eocene Yumaque formation. T h e second m a j o r Cenozoic s e q u e n c e includes the u p p e r m o s t O l i g o c e n e to l o w e r middle M i o c e n e C h i l c a t a y f o r m a t i o n w h i c h c o n s i s t s of s e v e r a l fining u p w a r d s cycles of i n d u r a t e d s i l t s t o n e s a n d s a n d s t o n e s . This u n i t contains abundant secondary carbonates, minor phosphorites and interbedded diatomac e o u s sediments. T h e y o u n g e s t s e d i m e n t a r y s e q u e n c e is c o m p r i s e d of t h e P i s c o F o r m a t i o n , of l a t e M i o c e n e t h r o u g h P l i o c e n e age. T h e Pisco F o r m a t i o n e x h i b i t s a diverse v a r i e t y of lithologies i n c l u d i n g c o n g l o m e r a t e s , tuffaceous s a n d s t o n e s a n d siltstones, d i a t o m i t e s , secondary carbonates, and phosphorites. M a j o r h i a t u s e s in the Pisco B a s i n s p a n the l a t e C r e t a c e o u s - e a r l y Eocene, e a r l y to late O l i g o c e n e ( ~ 36 24 Ma), m i d - M i o c e n e ( ~ 16 11 Ma), t h e l a t e P l i o c e n e - R e c e n t ( < 2 Ma; although Pleistocene marine terraces occur a l o n g the coast: M a c h a r 6 , 1987; Hsu, 1988). Minor hiatuses have been recognized within the l o w e r M i o c e n e a n d u p p e r M i o c e n e - P l i o cene sequences, h o w e v e r , t h e i r ages a n d regional e x t e n t are n o t well k n o w n . All t h r e e m a j o r t r a n s g r e s s i v e u n i t s w e r e influenced by s i g n i f i c a n t b a s e m e n t relief a n d e a c h s e q u e n c e h a s p r e s e r v e d a s i m i l a r facies p r o g r e s s i o n from n e a r s h o r e c o a r s e b i o c l a s t i c u n i t s to d e e p e r w a t e r o r g a n i c - r i c h facies.

258

Sediments in the Sechura Basin were deposited during 4 major transgressive events including the upper Eocene Chira/Verdun Fms., lower Miocene Manorca/Heath Fms., mid to upper Miocene Montera/lower Zapallal Fms., and the upper Miocene upper Zapallal Formation. Each sequence generally fines upwards. Hiatuses in the Sechura Basin span the early Paleocene to Eocene, Oligocene ( ~ 37-24 Ma?), early to mid Miocene ( ~ 18-15 Ma?), and late Miocene ( ~ 8-7 Ma). Biogenic enrichment is found in all three Pisco Basin sequences, manifested by the occurrence of organic dolomites, phosphorites, and diatom-rich sections. The highest concentrations of phosphorites are found in uppermost Miocene sections. Intervals of biosiliceous deposition in the Pisco Basin occurred at about 40-36 Ma, 24-16 Ma, and 11-3 Ma, with the most diatomaceous units occurring in the Late Eocene and Late Miocene through Pliocene. Diatomaceous sediments accumulated in the Sechura Basin between about 37 40 Ma and 8.5-4.5 Ma. Upper Eocene diatomites suggest that high productivity conditions linked to coastal upwelling have occurred episodically during the past 40 million years. Flora and fauna from the lower Miocene siliceous deposits have a relatively warm water aspect. Late Miocene-Pliocene diatom and radiolarian assemblages include many Antarctic and cold water species, providing evidence for late Neogene cooling and intensification of the upwelling system. The occurrence of diatomites and phosphorites in Neogene sediments is diachronous within different areas of the Pisco Basin, between the Pisco Basin and Sechura Basin, and between the Peruvian forearc deposits and the Monterey Formation of California. It is likely that a complex and variable history of uplift and subsidence has been a major control on the accumulation of biogenic sediments in Peruvian forearc basins. The volume of Neogene sediment in the central Peruvian forearc is nearly twice that of the Monterey Formation of California. Although specific intervals of rapid organic carbon deposition may not be synchronous

R.B. DUNBARET AL.

across the forearc, these Latin American deposits most likely contributed to postulated Miocene Co 2 excursions.

Acknowledgements This work was supported by National Science Foundation grant EAR 85-16528 to RBD and PAB. We thank Petroleos del Peru for their assistance with many aspects of the field work; special thanks go to Ings. Jos6 Zegarra, Jos6 Cruzado, and Ms. Betty Roy. Marian Allen, Carrie Stock, Sunjay Talwani, Robyn Wright, Jon Martin, Stephen Burns, and Rolando Cruzado assisted with field work. We thank John Barron for providing helpful comments on the manuscript and figures. R. Wright assisted with the stratigraphic descriptions presented here. M. Allen compiled the geologic map in Fig.3. We thank Hans Schrader (University of Bergen), Tom DeVries (U. South Carolina), B. Kues (U. New Mexico), R. Rosen (ARCO), and R. Arends, H. Heitman, M. Filewicz, and G. Blake (all at UNOCAL) for providing biostratigraphic information on Pisco Basin samples.

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