MARINE DEPOSITS OF CALETA PATILLOS, NORTHERN : THEIR RELATIONSHIP WITH NEOGENE SEDIMENTS OF THE PENINSULA DE MEJILLONES

HUMBERTO PADILLA(tl Empresa Nacional del Petróleo, ENAP, Casilla 3556, Santiago, Chile SARA ELGUETA

ABSTRACT

A 10 m thick marine sequen ce 01 Neogene age composed 01 terrigenoclastic sediments interbedded with bioclastic grainstones and coquinas and capped by diatomaceous calcareous mudstones has been recently recognized in Caleta Patillos (20"43'8),80 km south 01 Iquique. The organic Iramework 01 grainstones and coquinasconsists 01 molluscan, balanid and echinodenn fragments and foraminilera. Litho- and biolacies suggest deposition on an open shell basin next to a moderate coastal upwelling in an Andean lorearc domain. This sequence represents the northern extension 01 similar age deposits previously known Irom the Península de Mejillones, 250 km lurther south. This discovery partially confinns that Neogene marine sedimentation, including diatomaceous ocurrences, took place along the continental margin of Chile north 01 Mejillones, which up to now had been considered its northern limit, and probably represents the onland expression of a major depositional basin located mainly oftshore as is here demonstrated by the only seismic line available. In addition, it opens up possibilities 01 finding exposures of similar age between the above mentioned locations and the southern coastal Perú Tertiary sequences, both onshore and offshore.

Key worris: Dlatomaceous sediments, Neogene, Caleta Patillos, Penlnsula de Mejillones, Chile.

RESUMEN

DEPOSITOS NEOGENOS MARINOS DE CALETA PATlLLOS, NORTE DE CHILE: SU RELACION CON LOS SEDIMENTOS NEOGENOS DE LA PENINSULA DE MEJILLONES_ En Caleta Patillos (20°43'8), 80 km al sur de Iquique, aflora una secuencia marina neógena de 10 m de espesor, compuesta por sedimentos terrígeno-elásticos intercalados con calcaren itas bio­ elásticas y coquinas y fangolitas calcáreas diatomáceas en su parte más alta. La fracción orgánica de las calcaren itas y coquinas consiste en fragmentos de moluscos, balánidos y equinodennos, además de foraminíferos. Las lito- y biofacies encontradas penniten sugerir que la sedimentación tuvo lugar en una cuenca localizada principalmente en la platafonna externa, cercana al desarrollo de surgencias costaneras moderadas, en el dominio del antearco andino. Esta secuencia representa la extensión más septentrional conocida de unidades contemporáneas, expuestas 250 km al sur, en Península de Mejillones. Este hallazgo conlinna, parcialmente, la continuidad en la sedimentación marina neógena que incluye horizontes ricos en diatomeas a lo largo del margen continental del norte de Chile hasta ahora restringida a la latitud de Mejillones. La secuencia se encuentra en el extremo más oriental de una cuenca de sedimentación mayor ubicada, principalmente, mar aluera, como lo demuestra la línea sísmica disponible de este sector. Por otra parte, abre la posibilidad deencontrarnuevos afloramientos, similares a los mencionados, en localidades del norte del país y de establecerun vínculo más directo con las cuencas terciarias del sur del Perú.

Palabras claves: Sedimentos diatomáceos. Neógeno, Caleta Patillos, Penfnsula de Mejillones, Chile.

Revista Geológica de Chile, Vol. 19, No. 1, p. 83-89. 2 Figs. Julio 1992. 90· 60· O>... .)1 70° i'//Á ~y;;, ~ @@ I

O· ~7~,,·.--,. <:l AMERICA ..- ! PATI LLOS PISCO BASIN '. 2.0~45·S 1I I ~ ~ : I . PATlL t:;:g SEISMICILlNE 001·010 MEJILLONES · /0 ~ I ~. . O' . 5 'lO 'un 30· -j30· ~ COQUIMBO $ / / , / We'5tern edqe of I.J ~ ~ "'''( '- &~ ~i ".'.'" "." ... () I , " N 21° S I.J " ?1 BASEMENT I z ~ '- HEIGHTS '< I ~ 60· "( " ~60· G> 'l ,," mz ~ ~ Jurassic volcanic rocks Neogene sediments m ;:: ;1>­ ~ Jurassic plutonic rocks ~ ~ Coastal plain aolian sands Jl Z 90· 60· - m O m iJ S. O BO a, 90 OS 110 ,)5 , ,os 170 ,oa 200 zas 210 21S 213 . '00 '0' 1I~20 '15 '''' .«> .. .SO '75 'BS '''' ". en 000 ~ O "(') ~ ~ iJ ~ 1.00 .en5 z ::¡O ffi Jl Z 2.00 (') J: F Fig. 1. Location map of Caleta Patillos area and multichannel seismic line 001-010. H. Padilla and S. Elguela 85

INTRODUCTION

Up to the present, Neogene marine biogenic exposure 01 a similar marine sedimentary sequence siliceous sediments have been reported to occur in at Caleta Patillos, located on the coast at latitude northern Chile in three major sedimentary basins: in 20 0 43'S, 80 km south of Iquique and 250 km north of the Mejillones (23°15'S), Caldera (2]o05'S) and the Península de Mejillones (Fig. 1). Dne sample Coquimbo (300 00'S) basins. Siliceous fossil collected from this sequence contains a diverse and assemblages in these sediments consist of diatoms, moderately well preserved assemblage of diatoms silicoflagellates, sponge spicules, radiolarians and which are closely related to those described in the dinoflagellates. They form discrete beds up to 3 m Caleta Herradura Formation at Herradura Bay on the thick intercalated within bioclastic calcareous and western slope of Península de Mejillones (Tsuchi et terrigenoclastic sediments and locally, beds bearing al., 1988). Caleta Patillos is of considerable interest the diagenetic mineral assemblage phosphate, because it is the first documented ocurrence of dolomite and glauconite, and evaporites such as Neogene diatomaceous marine sediments between gypsum. Coquinas and bioclastic sandstones include Mejillones and the southernmost part of the Pisco a variety of species of planktonic and benthic Basin in Perú, a distance of about 800 km. foraminifera and nannoplankton. Foraminifera and Analyses of a seismic líne of this area revealed diatoms indicate an age ranging from middle Miocene an important sedimentary basin located in the shelf, to Pliocene for these sediments (Martínez, 1980; extending virtually from the continental margin to the Martínez and Caro, 1980; Tsuchi et al., 1988; Krebs coastline for approximately 50 km (Fig. 1). The basin et al., 1989). sediments lie directly and contains 1,200 m of Neogene to Recent sediments unconformably on Cambrian to Jurassic basement Iying on Jurassic basement rocks and in waterdepths rocks (Baeza and Venegas, 1988). Although between 0-100 m. Data on líthology and bio- and biostratigraphic work is available to some extent, the chronostratigraphy obtained from studies carried out complex sedimentaryfacies distribution in these areas on the sequence exposed onland at Caleta Patillos and their depositional and tectonic environments give us an insight into the sediments that may be have not been fully studied. infilling the basin. This paper reports the discovery of an isolated

GEOLOGIC SETTING

The NeogElne sequence observed at Caleta north and east as they overlap the basement rocks. Herradura and other places of the Península de They represent the nearshore uppermost part 01 an Mejillones are a, wedges gradually out to the north approximately 1,200 m thick sequence that is in1illing over a distance of 50 km. Most cliffs along the coast the offshore and slope basins, as shown by the and outcrops on the Coastal Plain from Mejillones to multichannel seismic line D01-01 O. Caleta Patillos are composed of Jurassic volcanic Sediments were deposited in the Andean forearc and plutonic rocks of the La Negra magmatic arc. basin domain which has characterized the continen­ These rocks form the main lithologies of the Cordille­ tal margin of Chile and Perú since times ra de la Costa in most parts of northern Chile (Mapa (Moberly et al., 1982). In contrast to southern Perú Geológico de Chile, 1982). They are almost entirely where the shelf is sufficiently wide to hold several covered by Recent eolían sands and silts, bioclastic basins (Dunbar et al., 1990), the Chilean shelf in the sands and coquina beaches. northern part of the country is very narrow, only 20- Neogene sediments at Caleta Patillos be long to 50 km wide. Up to date, the only Cenozoic basins on an unnamed formation and rest unconformably on the shelf with possibilities of having significant the Jurassic volcanic rocks. Sediments have a sequences are the offshore extensions of those maximum thickness of 10m measured in a low where the Mejillones, Caldera and Coquimbo coastal cliff, and are observed to pinch out to the sediments were deposited. According to Kulm et al. ""a>

LlTHOLOGY MICROFACIES MICROFOSSILS AGE

Coscinodiscus excentricus 10m. eolion sonds C. alf. C. miocenica C. cf. C. endoi ~UNCONFORMITY PLIOCENE C. el. C. margina tus TO Nitzchia el. N. reinholdii fine sondslone ond .illslone" Silty diolomoceous Nitzchia sp. LATE MIOCENE 503 micrilic ",ockestone Synedra el. S. platensis dlotoms - beorinll beds .a

z medium Ilroined sondstone m O (;) mz bioclostic sondslone m s ~» JJ Z hord biocloslic colcorenile beds m o m Textularia sp. (3 U'l Rotalidae inde!. Coorse orained sondy ¡;j 501 Lituolidae inde!. bioclastic IlroinSlone O Lenticulina~ike lagenids "TI (') coorse sondotone ~ ~ 50Z Coorse grained sondy 'U bioclostic grainstone Textularia sp. ~ molluscon ond balan id coquina 5 coorse sondstone !Jl O z O ~ J: m JJz (') J: F Fig. 2. Stratigraphic section of Neogene sediments at Caleta Patillos. H. Padilla and S. Elgueta 87

(1977), the reason being that in northern Chile the structures. Only a few isolated and starved subducting oceanic plate is broken into high relief sedimentary basins developed preferentially within extensional blocks and the forearc regio n is faulted basement in the middle of the slope, 30 to 90 characterized by steep trench walls, absence of km offshore in water depths of 900 to 1,200 m slope benches or basins, and truncated coastal (Coulbourn, 1981; Mordojovich, 1981).

L1THOLOGY, AGE AND BIOSTRATIGRAPHY

The 10m sequence exposed at Caleta Patillos is wackestone. Diatoms are poorly to moderately horizontal and composed of well bedded strata with preserved, generally broken and partially replaced a general upward fining character; at the base it by carbonate. Monoaxon sponge spicules were the consists o.f pale yellow coarse grained siliciclastic only other organic remainsfound. Diatoms determined sandstones interbedded with light gray molluscan were: and balan id coquinas. In the middle part of the section there are several hard bioclastic calcarenite Coscinodiscus excentricus Ehremberg beds and siliciclastic sandstones. Finally, at the top C. aft. C. miocenica Schrader there are fine grained sandstones, siltstones and C. cf. C. endoi Kanaya yellow terrigeneous diatomaceous beds (Fig. 2). C. cf. C. marginatus Ehremberg Three samples for petrographic and micropa­ Nitzchia cf. N. reinholdií Kanaya and Koizumi leontological work were selected from the most Nitzchia sp. biogenic calcareous and diatomaceous facies. Synedra cf. S. platensis Frenguelli Samples 501 and 502 are from a coarse grained Actynoptychus sp. sandy bioclastic grainstone composed of molluscan, Denticulopsis sp. balanid and echinoderm fragments and foraminifera Campylodiscus sp. tests; the terrigenoclastic component makes up 20% of the total grains and consists of quartz and volcanic According to M.C. López1, although the lithic fragments probably derived from the Jurassic foraminifera assemblage has a reduced chronologic basement rocks. A few small, structureless collophane value, the existing microflora suggests a late Miocene peloids are also presento Despite the coarse grain to Pliocene age for these sediments on the basis of size of these sedimentary rocks, the foraminifera are ranges of selected stratigraphically useful marine moderately well preserved, allowing the recognition planktonic diatoms given by Barron (1985). It of Textularia sp., Rotalidae indet., Lituolidae indet. corre lates well with the diatomaceous horizons and probably Lenticulina-like lagenids (M.C. López'). described by Tsuchi et al. (1988) and Krebs et al. Sample 503 is a silty diatomaceous micritic (1989) at Caleta Herradura de Mejillones.

PALEOENVIRONMENTAL SIGNIFICANCE

The microflora assemblage that characterizes transported to and trapped in a depression on the the upper part of the sequence is mainly composed open shelf during a maximum marine transgression of planktonic and neritic warm water forms with a in the Neogene. Their presence suggests that a minor non-marine component. Diatoms show strong diatom ooze was being deposited nearby in a high evidence of having been abraded, transported and productivity environment with moderate to deposited in an environment where calcareous intermittently strong upwelling and warm water sediments prevailed. influence. The presence of rare freshwater diatoms Diatomaceous sediments were probably may be due to river inflow.

'1989. Estado micropaleontológico de las muestras HPS·501, HPS·502 y HPS·503. Empresa Nacional del Petróleo (Inédito), 2 p. Santiago. NEOGENE MARINE DEPOSITS OF CALETA PATILLOS. NORTHERN CHILE ... 88

CONCLUSIONS

The small section of the Neogene sedimentary siliceous biogenic sediments may be present within sequence exposed at Caleta Patillos contains the this offshore sequence. typical facies and microfossiliferous assemblages Diatoms in Caleta Patillos sequence suggest a observed in other occurrences of similar age in late Miocene to Pliocene age for these sediments. northern Chile, that is: coarse terrigenoclastic This corre lates well with the Caleta Herradura sediments interbedded with molluscan and balanid diatomite exposed on the Península de Mejillones bioclastic sands and coquinas in the lower part of the (Tsuchi et al., 1988; Krebs et al., 1989). sequence and biogenic siliceous sediments in its This discovery shows that Neogene marine basins upper parto However, it neither contains the diversity and associated biogenic siliceous sediments in of siliceousflora northe diagenetic mineral association northern Chile have more continuous extension than present in the other locations. This may be due to the was thought in the past, and opens up possibilities of tact that sediments observed at Caleta Patillos finding other exposures or offshore sequences represent the nearshore uppermost part of a major between Península de Mejillones andthe Pisco Basin. sequence which is at present mainly offshore. More

ACKNOWLEDGMENTS

By the time this paper has been published, the seismic interpretation of the offshore basin, to V. main author, Humberto Padilla, passed away as the Covacevich (Servicio Nacional de Geología y Mine­ consequence of a fortuitous accident occurred in the ría) tor assisting in the final edition of this article and course offield work in the Arauco area. The co-author to H. Niemeyer (Universidad Católica del Norte), who (S. E.) hopes to have been able to reproduce as truly made significant suggestions to the original text. as possible H. Padilla's ideas on the Caleta Patillos Thanks are given as well to the Empresa Nacional del area. Thanks are given to E. Laval (Empresa Nacio­ Petróleo for support and permission to publish seismic nal del Petróleo) for his valuable cooperation in the section D01-01 O (Fig. 1).

REFERENCES

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Manuscript received: April29. 1991 ; accepted: March 13. 1992.